Place the tuna in a medium-size bowl and use a fork to break it into small pieces.
Add the onion, celery, gherkins, salt, pepper and mayonnaise with the tuna and stir until combined.
Spread the honey mustard on all bread slices. Top two slices of bread with one-half of the tuna. Lay the lettuce on top of the tuna, the tomatoes on top of the lettuce and sprinkle with a little salt and pepper. Cover, cut and devour!