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 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.
Reduce the heat to low, stir in the heavy cream, and let it warm through for 2 minutes. Do not let it boil after adding the cream.
Add the refrigerated tortellini to the simmering soup. 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, about 1 minute.
Serve the soup immediately.
Notes
For a heartier soup, substitute kale for spinach and cook it with the broth for about 5 minutes before adding the tortellini.
If you prefer a milder flavor, use sweet Italian sausage instead of hot.
You can use frozen tortellini; add them directly to the simmering broth and adjust the cooking time as needed.