Mix together the tuna, Miracle Whip, pickle and onion. Stir until blended. Toast bread in toaster oven so that both sides are lightly browned. Divide the tuna mixture evenly among the 4 slices of bread. Place a tomato slice on top of the tuna mixture and then top with your favorite cheese, as little or as much as you would like. Put back into the toaster oven and toast until the cheese is melted and bubbly. Makes 4 servings.