Heat a large skillet over medium heat. Add ground Italian sausage and cook for 6 to 8 minutes, breaking it apart, until browned with slightly crispy edges.
Add diced onion and sliced bell pepper to the skillet. Cook for 4 to 5 minutes until softened.
Stir in minced garlic and cook for about 30 seconds until fragrant.
Add tomato paste and cook for 1 minute to deepen the flavor.
Pour in crushed tomatoes and add Italian seasoning and red pepper flakes if using. Stir well and reduce heat to low.
Let the mixture simmer uncovered for 10 to 15 minutes, stirring occasionally.
Taste and adjust with salt and black pepper as needed.
Remove from heat and garnish with fresh basil or parsley.
Serve over pasta, rice, or mashed potatoes and top with grated Parmesan cheese.