In a small saucepan, cook sausage over medium heat until no longer pink; drain. Add onion; cook and stir until tender. Add garlic; cook 1 minute longer. Stir in broth and water; bring to a boil. Reduce heat; simmer, uncovered, for 10 minutes., Return to a boil. Reduce heat, add spinach and tortellini; cook until tortellini is tender, 7-9 minutes. Sprinkle with cheese. If desired, top with crushed red pepper flakes.