Begin by chopping the bell peppers and onion into bite-sized pieces, then mince the garlic cloves. Preheat your oven to 375°F (190°C).

Heat a large skillet over medium heat and add sliced Italian sausage. Cook for 5-7 minutes until browned and crispy.

Incorporate the chopped bell peppers and onions into the skillet with the sausage. Sauté for another 4-5 minutes until softened.

Stir in the minced garlic and diced tomatoes, letting the mixture simmer for 3-4 minutes.

Add Italian seasoning, along with salt and pepper to taste, stirring well. Transfer the mixture into a greased baking dish.

Place the casserole in the preheated oven and bake uncovered for 25-30 minutes until bubbly and golden brown.