Cut beef into 6 serving-size pieces. Layer half of the beef, mushrooms and onion in 3 1/2 to 4-quart slow cooker; repeat layers. Pour soup over ingredients in slow cooker. Cover; cook on low setting for 8 to 10 hours or until beef is tender and no longer pink.
Before serving, toss stuffing mix and contents of seasoning packet with melted margarine and 1/2 cup liquid from pot. Place stuffing on top of contents in slow cooker; cover.
Increase heat to high setting. Cook 10 minutes or until stuffing is fluffy. Sprinkle with cheese; cover. Cook until cheese is melted.