Place a large pot or Dutch oven over medium-high heat. Add the Italian sausage and cook, breaking it up with a spoon, until browned. Drain off excess grease.
Add the chopped onion to the pot and cook until softened, about 5 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Pour in the chicken broth and add the diced tomatoes (with their juice), Italian seasoning, basil, and red pepper flakes, if using. Bring the mixture to a simmer.
Add the refrigerated tortellini to the simmering broth. Cook according to package directions, usually about 5-7 minutes, until the tortellini float and are cooked through.
Reduce the heat to low. Stir in the heavy cream until fully combined and the soup is creamy. Do not let the soup boil after adding the cream.
Stir in the fresh spinach until it wilts into the soup.
Remove the pot from the heat. Stir in the Parmesan cheese until melted and smooth.
Taste the soup and add salt and black pepper as needed before serving hot.
Notes
For a heartier soup, add 1 cup of sliced mushrooms with the onion.
If you prefer a less rich soup, substitute half-and-half for the heavy cream.
You can use frozen tortellini; add them directly to the simmering broth, but you may need to increase the cook time slightly.