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. Add the minced garlic and cook for 1 minute more 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 medium-low, cover, and let it simmer for 10 minutes to allow the flavors to combine.
Stir in the heavy cream and bring the soup back to a gentle simmer. Do not allow it to boil rapidly after adding the cream.
Add the fresh spinach and stir until it wilts into the broth, about 2 minutes.
Add the refrigerated tortellini to the pot. Cook according to package directions, usually about 3 to 5 minutes, until the tortellini float and are heated through.
Remove the pot from the heat. Stir in the grated Parmesan cheese until melted and the soup is creamy. Serve immediately.
Notes
You can substitute ground chicken or ground turkey for the Italian sausage to make a lighter version.
If you prefer greens other than spinach, kale works well; add it a few minutes earlier than the spinach since it takes longer to soften.
This soup stores well in the refrigerator for up to 3 days. Reheat gently on the stovetop.