In a Dutch oven, cook sausage and onion over medium heat until sausage is no longer pink and onion is tender, 5-7 minutes, breaking up sausage into crumbles; drain. Add the garlic and pepper flakes; cook 1 minute longer. Stir in the broth, pasta sauce, crushed tomatoes, tomato paste, basil, vinegar, parsley flakes, sugar, oregano, salt and pepper; bring to a boil., Add tortellini; cook, uncovered, until tortellini are tender, 3-5 minutes, stirring occasionally. Serve immediately. If desired, top with cheese.