Cut beef into 6 serving-size pieces. Layer half of the beef, mushrooms and onion in 3.5 to 4 litre 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 half 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.