Drain the juice from the tuna, mash the fish with a fork. Cut the apple, onion and red pepper into small cubes. Mix with capers, tuna, lemon juice, mustard and mayonnaise.
Spread the salad on 6 slices of bread, cover with thin slices of cheese.
Put the bread in the pan, put on moderate heat. Close the lid and cook for 3-4 minutes until cheese is melted. Or bake a sandwich in the oven.