Preheat oven to 425°F.
Place bagels, cut side up, on foil-lined baking sheet pan.
Toast the bagels for 2-3 minutes or until they are light golden on the edges.
Combine tuna, 1 cup of cheese, apple and onion; mix well.
Evenly spread over sliced bagels.
Sprinkle with remaining cheese.
Bake for 6 to 8 minutes or until cheese is well melted.
