Heat the olive oil in a large Dutch oven or heavy-bottomed pot over medium heat.
Add the 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 diced tomatoes (with their juice). Add the Italian seasoning, basil, and red pepper flakes, if using. Bring the mixture to a simmer.
Add the refrigerated ravioli to the simmering broth. Cook according to the package directions, usually 5 to 7 minutes, until the ravioli float and are cooked through.
Stir in the fresh spinach until it wilts, about 1 minute.
Reduce the heat to low. Stir in the heavy cream and Parmesan cheese until the soup is creamy. Do not let the soup boil after adding the cream.
Taste the soup and season with salt and pepper as needed. Serve hot.
Notes
For a vegetarian version, omit the sausage and use vegetable broth. Add 1 cup of sliced mushrooms when you cook the onions.
If you do not have heavy cream, you can substitute it with 1/2 cup of half-and-half or 1/4 cup of cream cheese, softened, stirred in until melted.
This soup thickens as it cools. If you have leftovers, you may need to add a splash of broth or water when reheating.