Step 2
Coat grill rack with cooking spray; place on grill over medium-hot coals (350℉ to 400℉). Place meat patties and onion slices on rack; grill, covered, 5 minutes on each side or until meat is done. Place 1 pattie and 1 onion slice on the bottom half of each roll. If desired, add mustard and ketchup. Top with remaining roll half.