Preheat the oven to 375°F.
In a large skillet over medium heat, cook the ground beef or sausage until browned. Drain excess fat.
Add the onion, bell pepper, and garlic, cooking until softened, about 5 minutes.
Stir in the diced tomatoes, tomato sauce, oregano, basil, salt, and pepper. Simmer for 5 minutes.
In a greased 9x13-inch baking dish, layer half of the meat mixture.
Top with half of the pepperoni and half of the mozzarella cheese.
Repeat the layers with the remaining meat mixture, pepperoni, and mozzarella cheese.
Sprinkle the Parmesan cheese on top.
Bake in the preheated oven for 25-30 minutes, or until the cheese is bubbly and golden.
Let it cool for 5-10 minutes before serving. Garnish with fresh basil leaves if desired.