Slice each sausage link into 5 pieces. Heat 1 tablespoon oil in a large skillet over medium-high heat. Add sausage and cook until browned, turning as needed. Drain excess grease.

Add bell peppers, tomatoes, and Italian seasoning, then drizzle with remaining 2 tablespoons oil. Reduce the heat to medium-low, cover, and simmer for 15 minutes.

Stir in onion and garlic, then sprinkle with Parmesan cheese. Cover and simmer for 15 more minutes. Stir, then top with mozzarella cheese. Cover and cook for 5 minutes to melt the cheese. Remove from the heat and enjoy.