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 excess grease.
Add the chopped onion, carrots, and celery to the pot. Cook until the vegetables soften, about 5 to 7 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, oregano, and red pepper flakes if using. Bring the mixture to a boil.
Reduce the heat to low, cover, and simmer for 10 minutes to allow the vegetables to tenderize.
Increase the heat to medium-low. Add the refrigerated tortellini to the soup. Cook according to package directions, usually 3 to 5 minutes, until the tortellini float and are cooked through.
Reduce the heat to the lowest setting. Stir in the heavy cream and grated Parmesan cheese until the cheese melts and the soup is creamy. Do not let the soup boil after adding the cream.
Taste the soup and season with salt and black pepper as needed.
Serve hot.
Notes
For a thicker soup, you can add 1 tablespoon of cornstarch mixed with 2 tablespoons of cold water during the last 5 minutes of simmering.
Use fresh spinach or kale added at the very end for extra greens.
Substitute ground turkey or ground beef for the Italian sausage if desired.