1. Brown the sausage in a large heavy-bottomed pot over medium heat, breaking it into crumbles as it cooks, until no pink remains. Drain excess grease if needed, leaving about a tablespoon in the pot.

2. Add the olive oil if the pot looks dry, then stir in the chopped onion and cook until soft and translucent, about 4 to 5 minutes. Add the minced garlic and cook for 30 seconds, just until fragrant.

3. Sprinkle in the Italian seasoning, crushed red pepper flakes if using, 1/2 teaspoon salt, and a few grinds of black pepper. Stir to coat the sausage and onions in the seasonings.

4. Pour in the chicken broth and add the diced tomatoes with their juices, scraping along the bottom of the pot to release any browned bits. Bring the mixture to a gentle boil over medium-high heat.

5. Stir in the cheese tortellini, reduce the heat to maintain a lively simmer, and cook according to the package directions, usually 4 to 6 minutes, until the pasta is just tender.

6. Lower the heat to medium-low so the soup is barely simmering. Stir in the heavy cream and grated Parmesan cheese until the cheese melts and the broth becomes smooth and creamy. Do not let the soup boil hard.

7. Add the chopped spinach and cook for 1 to 2 minutes, just until wilted. Stir in the butter until melted, then taste and adjust seasoning with additional salt and pepper as needed.

8. Ladle the soup into bowls and serve hot, topped with extra Parmesan and black pepper if desired.