In a large pot, add the sausage, carrots, celery, onion, and garlic. Cook over medium heat, breaking sausage into small pieces. Cook until the sausage is no longer pink and the veggies are starting to soften. Drain excess grease.
Add the salt, basil, oregano, thyme, and pepper. Stir in the tomato sauce and broth.
Bring the soup to a simmer and cook for 5-6 minutes. Stirring occasionally to prevent sticking; moderate heat as needed.
Blend the milk and flour together until smooth. Whisk into the soup and bring to a simmer again. Cook for 2-3 minutes, stirring often, until slightly thickened.
Add the tortellini and spinach. Cook for 2-3 minutes until the tortellini is tender, stirring to prevent sticking. Add salt and pepper to taste, if needed.
Serve with freshly grated Parmesan to sprinkle on each serving, if desired. The soup will continue to thicken as it cools.
Notes
Substitutions/Variations: this soup can easily be doubled...or halved. You can skip the sausage (and just cook the veggies in a bit of olive oil) or sub in ground beef for the sausage. Kale would work great in place of the spinach. Small cheese ravioli could be subbed for the tortellini - or try another kind of unfilled pasta.