Heat the olive oil in a large pot or Dutch oven over medium-high heat. Add the Italian sausage and cook, breaking it up with a spoon, until browned. Drain excess grease.
Add the chopped onion to the pot and cook until softened, about 5 minutes. Add the minced garlic and cook for 1 minute more until fragrant.
Pour in the chicken broth and add the diced tomatoes (with juice), Italian seasoning, salt, and pepper. Bring the mixture to a boil.
Reduce the heat to medium-low, cover, and simmer for 10 minutes to allow flavors to combine.
Stir in the refrigerated tortellini and cook according to package directions, usually about 3 to 5 minutes, until tender.
Stir in the fresh spinach until wilted, which takes about 2 minutes.
Remove the pot from the heat. Stir in the heavy cream and grated Parmesan cheese until the soup is creamy.
Serve immediately.
Notes
For a richer flavor, use sweet or hot Italian sausage based on your preference.
If you do not have heavy cream, you can substitute with half-and-half, though the soup will be slightly thinner.
You can substitute kale for spinach if you prefer a heartier green.