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

1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Cook the Sausage: In a large skillet, heat olive oil over medium heat. Add the Italian sausage and cook until browned, breaking it apart with a spatula. This should take about 5-7 minutes.
3. Add Vegetables: Once the sausage is browned, add the sliced onion and bell peppers. Sauté for another 5 minutes until the vegetables are tender.
4. Add Garlic: Stir in the minced garlic and cook for an additional minute until fragrant.
5. Combine Ingredients: Add the diced tomatoes, tomato sauce, oregano, basil, and red pepper flakes (if using). Season with salt and pepper to taste. Stir until well combined.
6. Transfer to Baking Dish: Pour the mixture into a 9×13-inch baking dish, spreading it evenly.
7. Top with Cheese: Sprinkle the shredded mozzarella cheese evenly over the top of the casserole.
8. Bake: Place the baking dish in the preheated oven and bake for 25-30 minutes until the cheese is melted and bubbly.
9. Garnish: Remove from the oven and let it cool for a few minutes. Garnish with fresh basil if desired.
10. Serve: Slice and serve hot!

These steps will guide you in creating this incredible casserole effortlessly.