Cook the Sausage: In a large pot or Dutch oven, brown the Italian sausage over medium heat until fully cooked. If necessary, add olive oil to prevent sticking. Drain excess grease if needed.
Sauté Aromatics: Add the diced onion and garlic to the pot, sautéing until softened and fragrant (about 2 minutes). Stir in Italian seasoning and red pepper flakes.
Build the Base: Pour in the diced tomatoes (with juice) and chicken broth. Stir well, bringing the soup to a gentle simmer.
Cook the Pasta: Add the pasta directly into the simmering broth. Cook according to package instructions, stirring occasionally to prevent sticking.
Finish with Cream & Cheese: Lower the heat, then slowly stir in the heavy cream and shredded Parmesan cheese. Let the cheese melt into the broth for a rich, creamy texture.
Add Spinach & Season: If using, stir in the chopped spinach and let it wilt. Taste and adjust seasoning with salt and pepper as needed.
Serve & Enjoy: Ladle the soup into bowls, garnishing with fresh parsley and extra Parmesan. Serve warm with crusty bread.