Drain the tuna in a sieve. Mix yoghurt and salad cream. Season with salt and pepper. Pluck the tuna and stir into the cream. Wash and slice the tomato.
Wash the cucumber and cut into slices. Wash the salad and shake dry. Toast the slices of toast one after the other in a toaster. Spread 2 slices of toast with half the cream. Cover with lettuce, tomato and cucumber.
Spread the rest of the cream on top. Place one slice of toast on each, press down lightly and cut in half diagonally. Garnish with tomato wedges and parsley and arrange on a plate.