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, carrots, and celery to the pot. Cook until the vegetables soften, about 5 to 7 minutes.
Stir in the minced garlic, Italian seasoning, oregano, and red pepper flakes (if using). Cook for 1 minute until fragrant.
Pour in the chicken broth and add the diced tomatoes (with their juice). Bring the mixture to a simmer.
Reduce the heat to low. Stir in the ricotta cheese until it dissolves into the broth, creating a creamy base.
Stir in the heavy cream.
Add the refrigerated tortellini to the soup. Cook according to package directions, usually about 5 to 7 minutes, until the tortellini float and are cooked through.
Stir in the fresh spinach until it wilts completely.
Remove the pot from the heat. Stir in the Parmesan cheese. Season with salt and pepper to your preference.
Serve hot.
Notes
For a thicker soup, you can use half-and-half instead of heavy cream.
If you prefer a less spicy soup, omit the red pepper flakes.
You can substitute fresh kale for spinach; add it a few minutes earlier than the spinach to allow it time to soften.