Brown the sausage: Heat a large Dutch oven or heavy-bottomed soup pot over medium heat. Add the Italian sausage and cook for 6 to 8 minutes, breaking it into small crumbles with a wooden spoon or silicone spatula, until browned and fully cooked. Add olive oil if needed. Drain excess grease, leaving about 1 tablespoon in the pot.
Sauté the aromatics: Add the diced onion to the sausage and cook for 3 to 4 minutes, stirring occasionally, until softened. Stir in the minced garlic and cook for another 30 seconds until fragrant.
Add the seasonings: Stir in the tomato paste, Italian seasoning, and crushed red pepper flakes, if using. Cook for 1 to 2 minutes, stirring frequently, until the tomato paste darkens slightly and coats the sausage.
Build the broth: Pour in the diced tomatoes with their juices and the chicken broth. Stir well, scraping any browned bits from the bottom of the pot. Bring to a gentle boil, then reduce the heat and simmer uncovered for 8 to 10 minutes.
Cook the tortellini: Add the refrigerated cheese tortellini directly to the simmering broth. Stir gently and cook according to the package directions, usually 3 to 5 minutes, until the tortellini is tender.
Make it creamy: Reduce the heat to low. Slowly stir in the heavy whipping cream, followed by the freshly grated Parmesan cheese. Stir gently until the cheese melts and the broth becomes rich and creamy. Do not allow the soup to boil vigorously.
Add the spinach: Stir in the fresh baby spinach and cook for 1 to 2 minutes until wilted. Taste and season with salt and freshly ground black pepper as needed.
Serve: Ladle the creamy sausage tortellini soup into bowls. Garnish with additional freshly grated Parmesan and fresh basil or parsley. Serve warm with crusty bread or garlic bread.