Spray 3 1/2- to 4-quart slow cooker with cooking spray. Cut beef into 6 serving-size pieces. In cooker, layer half each of the beef, mushrooms and onion; repeat layers. Pour soup over top.
Cover; cook on Low heat setting 8 to 10 hours.
In medium bowl, mix stuffing mix, butter and 1/2 cup liquid from slow cooker; toss to mix. Place stuffing on top of mixture in cooker. Increase heat setting to High. Cover; cook 10 minutes longer or until stuffing is fluffy. Sprinkle with cheese. Cover; cook until cheese is melted.