Preheat your oven to 375°F.
In a large skillet, over medium heat, cook the ground beef or Italian sausage until browned, about 5 to 7 minutes. Drain excess fat.
Add the diced onion, bell pepper, and minced garlic to the skillet. Cook until softened, about 5 minutes.
Add crushed tomatoes, dried oregano, dried basil, salt, and black pepper. Stir and let simmer for about 5 minutes.
Transfer the mixture into a greased 9x13-inch baking dish. Spread evenly, then sprinkle mozzarella cheese on top, followed by pepperoni and Parmesan cheese.
Bake for 25 to 30 minutes until cheese is bubbly and golden brown. Let cool for 5 to 10 minutes before serving.