Preheat your oven to 375°F (190°C) and lightly grease a baking dish

Heat olive oil in a large skillet over medium heat

Add diced onion and cook until soft and translucent

Stir in minced garlic and sauté for 30–40 seconds until fragrant

Add ground beef or sausage and break it apart while cooking

Cook until fully browned and no pink remains

Drain excess grease to avoid an oily casserole

Add mushrooms and bell peppers to the skillet

Cook for 3–4 minutes until slightly softened but still firm

Pour in marinara or pizza sauce and stir well

Add Italian seasoning, salt, pepper, and chili flakes

Let the mixture simmer for 3–5 minutes to develop flavor

Remove from heat and allow it to cool slightly

Transfer the mixture into the greased baking dish

Spread evenly to create the base layer

Sprinkle half of the mozzarella cheese over the mixture

Add a layer of pepperoni slices evenly across the top

Sprinkle remaining mozzarella cheese and Parmesan cheese

Place the dish in the oven and bake for 20–25 minutes

Bake until cheese is melted, bubbly, and lightly golden

Switch to broil for 2–3 minutes for a crispy cheesy top

Remove from oven and let rest for 5 minutes before serving

Slice and serve warm