1In 5- to 6-quart slow cooker, place onion. Thoroughly rinse beef; discard seasoning packet. Place beef on onion; sprinkle with red pepper flakes. In small bowl, mix broth and Worcestershire sauce; pour over beef.
2Cover and cook on Low heat setting 8 to 9 hours or until beef is tender.
3Remove beef and onion from cooker, using slotted spoon. Cut beef across grain into thin slices. In small bowl, stir sour cream, horseradish and parsley until blended. Serve with beef and onion.