In a mixing bowl beat the cream cheese until smooth. Add the Parmesan, flour, bouillon, thyme, garlic powder, kosher salt, black pepper, and cayenne and mix until it is blended and smooth. Set aside.
For the Casserole Filling:
In a large high-sided skillet or Dutch oven, melt butter over medium-high heat. Add onion and mushrooms and cook, stirring occasionally, until onions are translucent and mushrooms have released their liquid and are just starting to caramelize, 7 to 8 minutes. Add the roasted red bell peppers and reserved gravy base. Add the milk and stir to combine into cream sauce. Stir until mixture starts to thicken, about 4 minutes, and remove from heat.
Stir in half of each cheese. Pour cheese sauce into a 9-by-13-inch baking dish or lasagna dish. Add the meatballs and tater tots, nestling them into the sauce and leaving some on top. Cover dish with foil and transfer to the oven.
Bake casserole until sauce is bubbling throughout, 45 to 50 minutes. Uncover and top with remaining cheese then bake until cheese is melted and top begins to caramelize, another 15 to 18 minutes.
Let cool slightly, then garnish with scallions or parsley and serve.
Video
Notes
•Find roasted red peppers in a jar at the grocery store. For extra flavor, (but hidden if the kids oppose!) I added sautéed mushrooms and roasted bell peppers, all topped with a golden blend of gouda and cheddar.