Preheat the oven to 450F/225C.
Saute the onions and garlic along with a pinch of red pepper flakes in the butter.
Push the veggies to one side and briefly stir fry the meat to brown all sides.
Season with salt, pepper and Italian seasoning.
Move the meat and veggies to the edges and add the mushrooms and tomato paste to the center. Bring everything together to coat with the tomato paste.
Transfer the mix to individual casseroles or a large baking dish and sprinkle the cheese on top.
Bake for 15-20 minutes or until the casserole turns golden brown. Alternatively, keep everything on top of the stove by reducing the heat under the skillet and topping with a lid until the cheese is melted.