In a large soup pot, cook the sausage over medium-high heat, stirring occasionally, until browned, about 7-10 minutes. Remove the sausage and set aside on a plate lined with paper towels, leaving about a tablespoon of fat in the pot. Drain any excess.

Add the finely chopped onion and celery to the pot, cooking for 4-5 minutes until softened. Stir in the minced garlic and flour, cooking for about one minute while constantly stirring.

Gradually pour in the chicken broth, stirring to ensure the flour dissolves completely. Add the diced tomatoes with their juices and Italian seasoning to the pot. Return the sausage to the mixture and raise the heat to bring the soup to a gentle boil.

Once the soup reaches a boil, add the tortellini and reduce the heat to maintain a gentle simmer. Cook for about 5 minutes until the tortellini is tender.

Stir in the heavy cream and continue cooking for another 5 minutes. Add the fresh spinach (if using) and season with salt and pepper to taste before serving.