olive oil • loose sweet Italian sausage or sausage links, casings removed (optional) • medium white or yellow onion, diced • dried fennel seeds • paprika • Salt and black pepper • crushed red-pepper flakes • tomato paste • garlic cloves, minced • can peeled tomatoes (crushed) or tomato purée • vegetable or chicken stock • (9- to 10-ounce) package refrigerated or frozen cheese tortellini •