Heat the olive oil in a large pot or Dutch oven over medium heat. Add the Italian sausage and cook, breaking it apart 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 simmer.
Add the refrigerated tortellini to the simmering broth. Cook according to package directions, usually about 5 to 7 minutes, until the tortellini float and are cooked through.
Reduce the heat to low. Stir in the heavy cream until fully combined.
Add the fresh spinach to the soup. Stir gently until the spinach wilts, which takes about 2 minutes.
Remove the pot from the heat. Stir in the grated Parmesan cheese until melted and the soup is creamy.
Serve immediately.
Notes
For a Tuscan flavor, add 1/2 cup sun-dried tomatoes (drained) along with the diced tomatoes.
If you prefer chicken, substitute the sausage with 1 pound of cooked, shredded chicken breast added during step 4.
You can use frozen tortellini; add them directly to the simmering broth and adjust cooking time as needed.