Cook the Ground Beef: In a large skillet, cook the ground beef over medium heat until browned. Drain any excess fat.
Add Vegetables: Add the chopped onion and minced garlic to the skillet. Cook until the onion is soft and translucent. Then, add the sliced mushrooms and cook until they are tender.
Prepare Sauce: Stir in the cream of mushroom soup and milk. Mix well until the sauce is smooth. Add salt, black pepper, paprika, and thyme (if using).
Combine Ingredients: If you want to include pasta or rice, mix it into the meat and mushroom mixture. Otherwise, you can proceed with the next step.
Layer in Baking Dish: Transfer the mixture to a greased 9×13-inch (23×33 cm) baking dish. Sprinkle the shredded cheddar and mozzarella cheeses evenly on top.
Optional Topping: If you like a crispy topping, mix the breadcrumbs with melted butter and sprinkle over the cheese.
Bake: Bake in the preheated oven for 20-25 minutes or until the cheese is bubbly and golden brown.
Serve: Let the bake cool for a few minutes before serving. Enjoy it as a standalone dish or with a side salad.