Heat a large skillet over medium-high heat, then add 1 pound sliced Italian sausage. Cook for 4-5 minutes, then remove the sausage from the pan and reserve it on a plate.
Add 1 tablespoon olive oil, 3 cups sliced bell peppers, and 1 cup sliced onion to the skillet. Cook until tender, about 5-6 minutes. Add 1 tablespoon minced garlic and cook for 1 minute.
Add sausage back to the skillet, along with 1 tablespoon Italian seasoning, 1 (15-ounce) can diced tomatoes, and 1 (8-ounce) can tomato sauce. Season with salt and ground black pepper to taste. Let it simmer for about 5 minutes, until heated through. Serve warm.
