Set oven control to broil. On cookie sheet, place bagels, cut sides up. Broil with tops 4 to 5 inches from heat 1 to 2 minutes or until desired brownness.
In medium bowl, mix tuna, mayonnaise, mustard and onions. Divide tuna mixture among 4 bagel halves. Top with apple and cheese slices.
Broil 1 to 2 minutes longer or until cheese is melted and tuna is warm.