Brown the Italian sausage in a large pot or Dutch oven over medium heat. Break the sausage into small pieces as it cooks. Drain off any excess grease.
Add the chopped onion to the pot with the sausage 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, salt, and pepper. Bring the mixture to a simmer.
Add the refrigerated tortellini to the simmering soup. Cook according to package directions, usually about 5 to 7 minutes, until the tortellini float and are tender.
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 grated Parmesan cheese until melted and smooth.
Serve immediately for a comforting dinner.
Notes
For a richer flavor, use sweet or hot Italian sausage based on your preference.
If you prefer kale over spinach, add chopped kale when you add the broth and simmer until tender before adding the tortellini.
This soup reheats well; if reheating, add a splash of extra broth or cream to restore the desired consistency.