Heat olive oil in a large pot or Dutch oven over medium heat. Add sausage and cook, breaking it up with a spoon, until browned. Drain excess grease.
Add onion, carrots, and celery to the pot. Cook until vegetables soften, about 5 to 7 minutes.
Stir in garlic and cook for 1 minute until fragrant.
Pour in chicken broth and add diced tomatoes (with juice), Italian seasoning, salt, and pepper. Bring the mixture to a boil, then reduce heat and simmer for 10 minutes.
Add the refrigerated tortellini to the simmering soup. Cook according to package directions, usually about 3 to 5 minutes, until the tortellini float.
Reduce heat to low. Stir in heavy cream and Parmesan cheese until the soup is heated through and creamy. Do not boil after adding cream.
Taste and adjust seasoning if needed. Serve hot, garnished with fresh parsley.
Notes
For a vegetarian version, omit the sausage and use vegetable broth, adding mushrooms or spinach for texture.
If you prefer a thicker soup, mix 1 tablespoon of cornstarch with 2 tablespoons of cold water and stir into the simmering broth before adding the tortellini.
You can use frozen tortellini; add them directly to the simmering broth and cook until tender.