1. Preheat broiler if manufacturer directs. Place beef top round steak on rack in broiling pan. Broil steak about 20 minutes for rare or until of desired doneness, turning steak once.
2. Meanwhile, in small bowl, mix mayonnaise, bread crumbs, mustard, and onion salt. When steak is done, spread mayonnaise mixture over top of steak; broil 2 minutes longer or until topping is hot and bubbly.