Divide the cheese among half the bread slices. Place the cheese and bread slices in the toaster oven and toast until the cheese is melted and the bread is lightly browned. Spread the other slices of bread with mayonnaise. Divide the turkey, bacon, apple, and onion between the mayonnaise slices of bread. Place in the toaster oven and toast until the bread is lightly browned and the other ingredients are warmed. Top the sandwiches with the remaining toasted cheesy bread halves and serve.