Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.

Cook the rotini pasta according to package instructions until al dente. Drain and set aside.

In a large skillet over medium heat, cook the ground beef or Italian sausage until browned. Drain any excess fat.

Add the diced onion, minced garlic, and bell pepper to the skillet. Sauté for about 5 minutes, or until the vegetables are tender.

Stir in the pizza sauce, diced tomatoes, Italian seasoning, salt, and black pepper. Bring to a simmer and cook for 2-3 minutes.

In a large bowl, combine the cooked pasta with the meat and sauce mixture. Mix well to ensure the pasta is evenly coated.

Pour half of the pasta mixture into the prepared baking dish. Sprinkle with 1 cup of mozzarella cheese and half of the pepperoni slices. Add the remaining pasta mixture on top, then sprinkle with the remaining mozzarella cheese, pepperoni, and Parmesan cheese.

Cover the dish with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.

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