Preheat the oven to 450°F (225°C).
Slice or chop the mushrooms.
Chop the onion and bell pepper finely.
Fry the vegetables in butter until slightly soft.
Put aside.
Slice the meat and fry in the same frying pan.
Add the garlic and spices.
Season with salt and pepper.
Return the veggies to the pan and stir.
Place everything in a greased baking dish and sprinkle the cheese on top.
Bake in the oven for 15-20 minutes or until the casserole turns golden brown.
Drizzle marinara sauce on top and serve with leafy greens and olive oil.