First, grease a 9x13 inch casserole dish and preheat the oven to 350 degrees.
Place a large non-stick pan over medium heat and add butter and melt, add onion slices and cook for 1-2 minutes, until just tender.
Add minced garlic, asparagus, mushrooms, and cook, stirring occasionally, for about 6-8 minutes.
Season with salt, pepper, paprika, thyme, and lemon zest. Stir to combine.
In a measuring cup combine cream and flour, whisk to combine.
Add the flour mixture to the skillet and cook, stirring for 3-4 minutes, or until thickened.
Reduce heat to low and mix in mayonnaise and cream cheese, stir until fully combined.
Remove from heat and stir in both kinds of cheese until melted.
Crush about 1 1/2 cups of french-fried onions and mix them into the asparagus mixture.
Transfer mixture to a prepared casserole dish.
Sprinkle remaining french-fried onions on top.
Place in oven and bake for 30-40 minutes or until mixture is bubbly.
Top with more french-fried onions and serve.