Preheat the oven to 375°F.

In a large skillet over medium heat, brown the ground beef or sausage until fully cooked. Drain any excess fat.

Add the diced onion, bell pepper, and minced garlic to the skillet, cooking until the vegetables are softened, about 5 minutes.

Stir in the crushed tomatoes, oregano, basil, salt, and pepper. Simmer for 5 minutes.

In a greased 9x13-inch baking dish, layer half of the meat mixture on the bottom.

Sprinkle half of the mozzarella cheese over the meat.

Add half of the pepperoni slices on top.

Repeat the layers with the remaining meat mixture, mozzarella cheese, and pepperoni.

Top with the grated Parmesan cheese.

Bake in the preheated oven for 25-30 minutes, or until the cheese is bubbly and golden.

Let cool for 5 minutes before serving. Garnish with fresh basil leaves if desired.