Heat the olive oil in a large pot or Dutch oven over medium heat. Add the Italian sausage and cook, breaking it up with a spoon, until browned. Drain off any 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, salt, and pepper. Bring the mixture to a boil.
Reduce the heat and simmer for 10 minutes to allow the flavors to combine.
Add the refrigerated tortellini to the simmering soup and cook according to package directions, usually about 3 to 5 minutes, until they float.
Reduce the heat to low. Stir in the heavy cream and Parmesan cheese until the cheese melts and the soup is creamy. Do not let the soup boil after adding the cream.
Stir in the fresh spinach until it wilts, about 1 minute.
Serve immediately.
Notes
You can substitute mild Italian sausage for spicy if you prefer less heat.
For a thicker soup, mix 1 tablespoon of cornstarch with 2 tablespoons of cold water, then stir into the simmering broth before adding the cream.
Use frozen tortellini if preferred; add them directly to the simmering broth and cook until heated through.