Crumble sausage into a Dutch oven; add onion. Cook and stir over medium heat until meat is no longer pink. Add garlic; cook 1 minute longer. Stir in the broth, water and tomatoes. Bring to a boil.
Add tortellini; return to a boil. Cook for 5-8 minutes or until almost tender, stirring occasionally. Reduce heat; add the spinach, basil, pepper and pepper flakes. Cook 2-3 minutes longer or until spinach is wilted and tortellini are tender. Serve with cheese if desired.
Freeze option: Place individual portions of cooled soup in freezer containers and freeze. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally; add broth if necessary.