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 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 juice), Italian seasoning, salt, and pepper. Bring the mixture to a simmer.
Add the refrigerated tortellini to the simmering soup. 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 and grated Parmesan cheese until the cheese melts and the soup is creamy. Do not boil after adding the cream.
Stir in the fresh spinach until it wilts, about 1 minute.
Serve hot.
Notes
For a spicier soup, use hot Italian sausage instead of mild.
If you prefer a tomato-free version, omit the diced tomatoes and add 1/2 cup tomato paste with the broth.
You can substitute ground turkey or chicken for the sausage if desired.