Brown the Italian sausage in a large pot or Dutch oven over medium heat. Drain 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, Italian seasoning, salt, and pepper. Bring the mixture to a simmer.
Add the refrigerated tortellini to the simmering broth. Cook according to package directions, usually 5 to 7 minutes, until the tortellini float and are cooked through.
Stir in the fresh spinach until it wilts completely.
Reduce the heat to low. Slowly stir in the heavy cream and Parmesan cheese until the soup is creamy and heated through. Do not boil after adding the cream.
Serve immediately.
Notes
For a thicker soup, you can mix 1 tablespoon of cornstarch with 2 tablespoons of cold water and stir this slurry into the simmering soup before adding the cream.
Use sweet or hot Italian sausage based on your preference.
If you do not have fresh spinach, use frozen spinach, thawed and squeezed dry.