In a large Dutch oven or heavy-bottomed pot, cook the Italian sausage over medium heat, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes.
Add the diced onion, carrots, and celery to the pot with the sausage. Cook, stirring occasionally, until the vegetables are softened, about 5 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Pour in the chicken broth and add the diced tomatoes with their juices, dried Italian seasoning, crushed red pepper flakes, salt, and black pepper. Stir to combine.
Bring the soup to a boil, then reduce the heat to medium-low and simmer for 10 minutes to allow the flavors to meld and vegetables to soften further.
Add the cheese tortellini to the pot and cook according to package instructions, usually 3-5 minutes, until the tortellini float to the top and are tender.
Reduce the heat to low and stir in the heavy cream and chopped spinach. Cook for 2-3 minutes until the spinach wilts and the soup is heated through.
Remove the pot from heat and stir in the grated Parmesan cheese until melted and incorporated.
Taste and adjust seasoning with additional salt and black pepper if needed.
Ladle the soup into bowls and serve immediately.