Preheat oven to 375F.
Lightly grease a 9x13 inch baking dish with cooking spray. Set aside.
Brown the ground beef in a large skillet over medium-high heat. Season with salt and pepper and add Worcestershire sauce.
Drain excess fat and transfer cooked beef to the bottom of the prepared baking dish. Distribute evenly.
Add the onions and mushrooms to the skillet and sauté for 4 to 5 minutes until translucent.
Add the garlic and cook, stirring frequently, for an additional minute.
Season with salt and pepper to taste.
Add flour and stir to combine to make a roux.
Add heavy cream little by little and stir to combine. Taste and adjust for salt and pepper. Let it simmer for 1 minute.
Reduce heat to low and stir in sour cream. Remove from heat and spread the mixture on top of the beef.
Top with 2 cups of cheddar cheese. Top with green beans and another 2 cups of cheddar cheese.
Finally, top with frozen tater tots arranging them one next to each other over the whole surface of the casserole.
Bake for 35 to 40 minutes or until tater tots are golden brown and cheese is melted.