Creating the Italian Sausage & Pepper Casserole is a straightforward process. Follow these steps for a delicious result:

1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Cook Sausage: In a large skillet over medium heat, cook the Italian sausage until browned, breaking it apart with a spoon. Remove the sausage and drain excess fat if necessary.
3. Sauté Vegetables: In the same skillet, add sliced bell peppers and onion. Sauté for about 5-7 minutes until they are softened. Add minced garlic and cook for 1 more minute.
4. Combine Ingredients: Return the cooked sausage to the skillet. Add the diced tomatoes, marinara sauce, oregano, basil, salt, and black pepper. Stir until well combined.
5. Mix in Pasta: In a large bowl, combine the cooked pasta with the sausage and pepper mixture. Mix well to ensure the pasta is evenly coated.
6. Layer in Casserole Dish: Transfer half of the pasta mixture into a greased 9×13-inch casserole dish. Sprinkle half of the mozzarella cheese on top. Layer the remaining pasta mixture and finish with the remaining mozzarella and Parmesan cheese.
7. Bake: Cover the dish with aluminum foil and bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 15-20 minutes, until the cheese is bubbly and golden.
8. Let it Rest: Once baked, remove the casserole from the oven and let it rest for about 10-15 minutes to set.
9. Garnish: Optionally, sprinkle with fresh basil leaves before serving.

These steps will guide you in creating a fantastic Italian Sausage & Pepper Casserole that everyone will love!