In a large pot or Dutch oven over medium heat, crumble and cook the Italian sausage until browned and cooked through, about 5-7 minutes. Use a slotted spoon to remove the sausage and set aside, leaving the rendered fat in the pot.
Add the diced onion, carrots, and celery to the pot. Cook, stirring occasionally, until the vegetables are softened, about 5 minutes.
Add the minced garlic and cook for 1 minute until fragrant.
Pour in the chicken broth and diced tomatoes with their juice. Stir in the Italian seasoning, crushed red pepper flakes, salt, and black pepper.
Bring the soup to a boil, then reduce heat and simmer for 10 minutes to allow the flavors to meld.
Add the cooked sausage back to the pot along with the cheese tortellini. Cook until the tortellini are tender, about 5 minutes.
Stir in the heavy cream and baby spinach. Cook until the spinach is wilted, about 2 minutes.
Remove the pot from heat and stir in the grated Parmesan cheese until melted and combined.
Taste and adjust seasoning with additional salt and pepper if needed.
Serve hot with extra Parmesan cheese on the side if desired.