Brown the Italian sausage in a large pot or Dutch oven over medium heat. 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, Italian seasoning, basil, and red pepper flakes. Bring the mixture to a simmer.
Reduce the heat to low, cover, and let the soup simmer for 10 minutes to allow the flavors to combine.
Increase the heat slightly and add the refrigerated tortellini. Cook according to package directions, usually 3 to 5 minutes, until the tortellini float.
Stir in the heavy cream and fresh spinach. Cook just until the spinach wilts, about 2 minutes. Do not let the soup boil after adding the cream.
Remove the pot from the heat. Stir in the grated Parmesan cheese until melted and the soup is creamy.
Taste the soup and adjust seasoning with salt and pepper as needed before serving.
Notes
For a thicker soup, you can whisk 2 tablespoons of cornstarch with 1/4 cup of cold water and stir this slurry into the simmering broth before adding the tortellini.
Use mild or hot Italian sausage depending on your preference for spice.
If you do not have heavy cream, half-and-half can substitute, but the soup will be slightly less rich.