Set oven control to broil. Spray broiler pan rack with cooking spray.
In medium bowl, mix egg, milk, mustard, salt and pepper. Stir in bread crumbs and onion. Stir in beef. Shape mixture into 6 patties, each about 1/2 inch thick.
Place patties on rack in broiler pan. Broil with tops about 5 inches from heat about 10 minutes for medium, turning once, until meat thermometer inserted in center reads 160°F. Serve burgers on buns.