Peel and finely chop onion and gherkins. Drain tuna and mix with onion, gherkins and mayonnaise in a bowl. Cut Tasty Toms into slices. Spread 4 slices of bread with the tuna salad. Cover each slice with another slice of bread. Top this bread with Tasty Tom slices.Cover with remaining slices and thickly sprinkle with cheese.Place sandwiches into oven for approx. 5-8 minutes until cheese starts to melt and turns bubbling brown.