Creating the Italian Sausage & Pepper Casserole can be straightforward if you follow these simple steps:

1. Preheat the Oven: Begin by preheating your oven to 375°F (190°C).
2. Cook the Sausage: In a large skillet over medium heat, brown the Italian sausage. Break it apart with a spatula until fully cooked, about 5-7 minutes.
3. Add Vegetables: Stir in the sliced onion, bell peppers, and minced garlic. Cook for an additional 5-7 minutes until the vegetables soften.
4. Season Mixture: Add the diced tomatoes, oregano, basil, red pepper flakes, salt, and pepper. Stir to combine and let it simmer for 5 minutes.
5. Combine with Pasta: In a large mixing bowl, combine the cooked pasta with the sausage and vegetable mixture. Mix well to ensure the pasta is evenly coated.
6. Transfer to Baking Dish: Pour the mixture into a greased 9×13-inch baking dish, spreading it out evenly.
7. Add Cheeses: Sprinkle the shredded mozzarella and grated Parmesan cheese evenly over the top of the casserole.
8. Bake: Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the cheese is melted and bubbly.
9. Cool Slightly: Once baked, remove the casserole from the oven and let it cool for about 5-10 minutes before serving.
10. Garnish: Optionally, sprinkle fresh basil on top for an extra burst of flavor and color.

Following these steps will guide you in creating this incredible casserole effortlessly!