In a large skillet, cook beef and onion over medium heat until beef is no longer pink, breaking up beef into crumbles, 6-8 minutes; drain. Transfer to a 4- or 5-qt. slow cooker. Add potatoes, broth, milk, carrots, celery and seasonings. Cook, covered, on low until vegetables are tender, 7-9 hours., Stir in cheese until melted. Stir in sour cream. If desired, sprinkle with parsley.