Caramelized Onion Tart with Mushrooms
When it comes to elegant appetizers, I opt for my caramelized onion tart with mushrooms and cheese. This delicate and intricate tart never fails to impress everyone with its buttery, flaky crust, which hosts a decadent, tender onion and mushroom mixture infused with brown sugar and fresh herbs. This simple yet sophisticated recipe is my secret to elevating any dinner party.
When it comes to hosting a larger crowd, I always plan and rely heavily on appetizers, especially finger food. Having a few options ensures that everyone will find something that they like. I make sure to include a few easy-to-make dishes, such as bacon-wrapped shrimp coated in brown sugar, and a super cheesy and gooey Philly cheesesteak dip with a variety of crunchy veggies and crackers.
Table of contents
Baking this French onion tart fills my kitchen with the most wonderful aromas, instantly predicting a good meal. I am a massive fan of it, and especially the option to make it in advance. Don’t you love recipes that look time-consuming, but end up being fuss-free and approachable? I sure do, and this is one of those. The perfectly caramelized onions, the slightly chewy mushrooms, the melted cheese, and the tender crust combine to create an irresistible blend of textures and flavors that brings comfort and approachable elegance to any meal.
Why you will love this recipe
- A classy appetizer: This is an easy yet classy appetizer, featuring a blend of woodsy and herby flavors. This French onion tart looks stunning and tastes exquisite, always becoming the talk of the dinner party.
- Made in advance: I love prepping as much as possible a few days in advance of hosting a party or dinner party. This homemade onion tart is no exception. I usually caramelize the onions and mushrooms a day in advance, and assemble the tart before the guests are scheduled to arrive.
- It’s made without meat: Perfect for vegetarians, this tart is rich and filling, thanks to the mushrooms and cheese. If you replace the beef stock with vegetable stock, it’s a delicious vegetarian appetizer.
- Unique and fun: If you are tired of the usual appetizers and are looking for something unique that will impress everyone, this is the recipe you should definitely try.
What you will need
- Pastry: I use a 14-ounce all-butter puff pastry (thawed according to the package directions) for a light, buttery, and flaky crust.
- Veggies: I prefer using organic mushrooms, but I try to get both cremini (also known as baby Bella) and shiitake so that I can have a mixture of the two flavors and textures. To go with the mushrooms, I add three onions that I caramelize with sherry, Worcestershire sauce, and brown sugar, then sprinkle with thyme, garlic, and rosemary.
- Cheese: Grated Gruyère cheese lends this tart a nutty and rich flavor, complemented by a creamy texture. I also like to sprinkle in some freshly shredded mozzarella cheese because it melts so smoothly and has a mild, creamy taste.
- Wet ingredients: The beef stock I use is a low-sodium variety, as I prefer to know the exact amount of salt going into my tart. Additionally, I add a bit of Worcestershire sauce to the beef stock, which lends it an extra layer of umami flavor. I use unsalted butter to sauté the onions and mushrooms because I prefer to control the amount of salt that goes into the recipe. Another liquid I add to make these veggies taste incredible is sherry, which caramelizes the onions and mushrooms. I also need an egg to use as egg wash for the crust.
- Seasonings: I add a hint of brown sugar to the sauce for a rich, sweet flavor. I also use salt and black pepper to taste.
- Herbs and spices: The chopped fresh rosemary enhances the mushrooms with a piney flavor and a vibrant taste. Fresh thyme leaves complement the richness and add brightness. The garlic adds a sharp yet sweet bite, providing a contrast that makes this tart stand out from the others.
How to make
Caramelize the onions: First, I caramelize the onions in 2 tablespoons of butter over medium-high heat, sprinkling with a pinch of salt, brown sugar, and pepper, for about 20-30 minutes, until they are jammy.
Liquids: Then, I add the sherry, Worcestershire sauce, and beef stock, turning the heat down to low.
Add the mushrooms: After letting it bubble for a minute, I remove the onions to a dish and add the rest of the butter, turning the heat back up to medium-high before adding the mushrooms to sauté for seven minutes, or until they are golden brown.
Herbs: They should still be meaty. Then, I add the garlic, rosemary, and one teaspoon of the fresh thyme, stirring until it is aromatic before turning off the heat. Now, I spoon the mushrooms into a bowl with a paper towel to absorb the excess moisture.
Preheat the oven: Meanwhile, I preheat the oven to 400°F and spread parchment paper sprinkled with flour on my working surface. Then, I roll out the puff pastry sheet into a rectangle that is about 1/8 inch thick. To make a border, I slightly score lines ½ inch from the edge around the sides. Then, I poke the center a few times with a fork to keep it from puffing up and brush it with egg wash to protect it from getting soggy.
Prep the tart: Next, I sprinkle 1 cup of the cheese on the bottom, top with the mushrooms and onions, and then add another cup of cheese. I brush the edges of the tart with the remaining egg wash to give them a nicely browned and glossy finish.
Bake: Next, the tart is placed in the oven for 25 to 30 minutes, depending on its thickness and the amount of cheese on top.
Cool and serve: Afterward, it needs to cool on the baking sheet for 10 minutes before I sprinkle the remaining thyme leaves on top and drizzle with balsamic glaze to serve.
Expert tip
The best mushrooms to use for an onion mushroom tart
I like to use both shiitake and cremini mushrooms in this recipe because I appreciate their distinct flavors and textures. The shiitake mushrooms have a meaty, earthy taste with a chewy, firm texture that I can really sink my teeth into. Cremini mushrooms are softer and have a milder flavor that pairs well with almost anything, and they absorb juices more effectively. The most common mushrooms are the button (white) mushrooms that are almost always used in steakhouses because they are mild and absorb juices. They can be used well in this dish too.
More tips to consider:
- I like to make my caramelized onions and mushrooms the day before so they can cool, which saves time. They also have time to soak up the juices. Just be sure to let them come to room temperature before prepping the pastry.
- Don’t forget the egg wash for the crust to help it brown and to keep it from getting soggy.
- When slicing onions, cut them in half first, then peel the outer layer before slicing them again, making them easier to slice.
- Try not to move the vegetables around in the pan too much while they cook, so they brown evenly.
- Make sure the pastry is chilled. Warm pastry is too sticky to use.
Recipe variations and add-ins:
- Add meat: Sometimes, I like to add some meat to this tart, such as chopped grilled chicken, sausage, ground beef, or ham.
- Individual tarts: To make these handheld tartlets, I use cookie cutters or biscuit cutters to shape them into small circles before topping and baking.
- Other cheeses: I have also used shredded parmesan, cheddar, Swiss, and pepper jack.
- More veggies: I also like to add other vegetables, such as chopped bell peppers, olives, and tomatoes.
- Spicy Tart: Make this a spicy tart by adding chopped jalapenos and red pepper flakes.
Serving suggestions:
Suppose I am serving this as an appetizer. In that case, it pairs perfectly with a glass of white wine, such as a buttery Chardonnay or a citrusy, crisp Sauvignon Blanc, accompanied by some of my fried mozzarella cheese sticks on the side. Another appetizer that everyone loves is my Buffalo chicken crack dip. I serve this on game day, for barbecues, or any day I want a dip that pairs well with almost anything. It is made with ranch, shredded chicken, and cheddar cheese, along with bacon and a hint of buffalo sauce. It’s so addictive, I could eat it with a spoon!
Sometimes I make a few of these onion tarts and serve them as a meal, accompanied by a salad. My Italian chopped salad pairs perfectly with this tart and is also hearty, featuring ham, salami, bell peppers, black olives, tomatoes, and pine nuts. However, if I want a side dish for this tart, my brown sugar-roasted carrots are a good choice. They are sweet and caramelized with brown sugar, which pairs very well with the caramelized onions.
How to store leftovers:
- Refrigerate: I save my leftover onion tart in an airtight container in the fridge for up to a week.
- Freezing: To freeze, I wrap each piece separately in plastic and place it in a small freezer bag. They can be stored in the freezer for up to a month.
- Defrost: I thaw mine overnight in the fridge for the best texture and taste.
- Reheating: I reheat my tart in the air fryer at 325°F for 2 minutes to achieve a crispy crust. In the microwave, it does not get crispy.
Frequently asked questions
Too much moisture in the filling or underbaked pastry is the likely culprit. To prevent this, I precook the mushrooms and onions, so they do not lose any excess moisture when they bake. But be sure to absorb the excess moisture and oil after cooking them so they do not bring too much oil or water with them. Also, I brush the egg wash on the bottom of the crust to prevent any excess moisture from making the crust soggy. Just be sure to let the egg wash dry first.
It is likely due to excessive moisture from the onions or mushrooms, as mentioned in the question above. If not, it could be from not baking it long enough. If it is underbaked, the filling does not have enough time to set correctly, resulting in a watery texture. Overbaking can also be a problem, though. Be sure to keep an eye on the tart and remove it when it is just starting to brown. Then, let it cool properly so the filling has time to set, or it will be watery.
If you need your puff pastry and forgot to thaw it out, it doesn’t take very long. Just remove it from the freezer and let it thaw at room temperature for about 30 to 40 minutes. However, for a super-fast thaw, which I do not recommend, it can be microwaved. Place one sheet at a time in the microwave for 15 seconds, then flip it over and heat for an additional 15 seconds. Repeat in five-second increments until it can be unfolded but still feels cold.
These tarts can be made with any dough. However, since these are savory tarts, I do not recommend using a sweet dough, such as sugar cookie dough. Try a simple no-yeast dough made of two ingredients. It is made with one part self-rising flour and one part nonfat Greek yogurt. Just mix it and it is done. There is also pie dough that can be bought from the store.
More mouthwatering appetizers to try:
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Caramelized Onion Tart with Mushrooms
Ingredients
- 4 tablespoons unsalted butter divided
- 3 medium sweet onions thinly sliced
- Salt to taste
- Ground black pepper to taste
- 1 tablespoon brown sugar
- 1/4 cup sherry or dry white wine
- 1 tablespoon Worcestershire sauce
- 1/4 cup beef stock or veggie stock for vegetarian
- 8 ounces shiitake mushrooms thinly sliced
- 8 ounces baby Bella mushrooms thinly sliced
- 4 cloves garlic minced
- 1 teaspoon fresh rosemary leaves diced
- 1 tablespoons fresh thyme leaves divided use
- 1 14 ounces package puff pastry thawed according to package instructions
- 1 1/2 cups grated Gruyere cheese
- 1/2 cup shredded mozzarella cheese
- 1 egg whisked for egg wash
Instructions
Caramelize the onions:
- Place a large, heavy-bottomed skillet over medium-high heat and add 2 tablespoons of butter. Once the butter is melted, add the sliced onions and season them with salt, brown sugar, and ground black pepper.
- Cook the onions over medium heat, stirring frequently, until they are very tender, golden brown, caramelized, and jammy. This process takes about 20 to 30 minutes.
- Once the onions are caramelized, stir in the sherry and Worcestershire sauce, then bring the mixture to a quick, bubbly boil for one minute. Next, add the beef stock and bring it to a boil for another minute.
- Reduce the heat to low and transfer the onions into a bowl.
Cook the mushrooms:
- To the same skillet, add the remaining two tablespoons of butter and melt.
- Raise the heat to medium-high and add the sliced mushrooms, seasoning them with salt and pepper.
- Sauté the mushrooms for about 7 minutes until they are golden and still have a slightly chewy texture. Stir in the garlic, rosemary, and one teaspoon of fresh thyme, cooking for another minute or two before turning off the heat.
- Transfer the mushrooms into a separate bowl lined with paper towels to absorb excess moisture.
Assemble and bake:
- Preheat the oven to 400°F.
- Lay a piece of parchment paper on an 18 x 3-inch baking sheet.
- Unroll the puff pastry sheet onto a lightly floured surface, and gently press it with a rolling pin to even the edges and make it symmetrical. I recommend keeping it about ⅛” thick.
- Gently score the puff pastry about ½ inch from the edges to create a border. Using a fork to prick the center of the puff pastry a few times to help it bake flat. Chill the pastry for a few minutes if it is becoming warm.
- Evenly sprinkle 1 cup of shredded Gruyère cheese on the bottom of the puff pastry. Then, top it with the sautéed onions and mushrooms, followed by the remaining Gruyère and mozzarella cheese.
- Brush the border with egg wash, and bake the tart for 25 to 30 minutes in the preheated oven, or until golden brown.
Serve:
- Allow the onion mushroom tart to cool on the baking sheet for 10 minutes. After that, garnish with the remaining two teaspoons of thyme leaves, slice it, and serve.
Notes
The best mushrooms to use for an onion mushroom tart
I like to use both shiitake and cremini mushrooms in this recipe because I appreciate their distinct flavors and textures. The shiitake mushrooms have a meaty, earthy taste with a chewy, firm texture that I can really sink my teeth into. Cremini mushrooms are softer and have a milder flavor that pairs well with almost anything, and they absorb juices more effectively. The most common mushrooms are the button (white) mushrooms that are almost always used in steakhouses because they are mild and absorb juices. They can be used well in this dish too.More tips to consider:
- I like to make my caramelized onions and mushrooms the day before so they can cool, which saves time. They also have time to soak up the juices. Just be sure to let them come to room temperature before prepping the pastry.
- Don’t forget the egg wash for the crust to help it brown and to keep it from getting soggy.
- When slicing onions, cut them in half first, then peel the outer layer before slicing them again, making them easier to slice.
- Try not to move the vegetables around in the pan too much while they cook, so they brown evenly.
- Make sure the pastry is chilled. Warm pastry is too sticky to use.