Cook the egg in boiling water for about 10 minutes. Drain the tuna. Wash the salad and pat dry. Clean, wash and thinly slice the tomato. Peel onion and cut into rings.
Toast bread. Season yoghurt with salt, pepper and lemon juice. Quench egg cold, peel and slice. Cut the crust off the bread. Cover one slice with salad and sprinkle with yoghurt.
Spread the tuna, onion rings and tomato on top and cover with egg slices. Cover with the rest of the bread and press down lightly. Cut bread diagonally in half, arrange and garnish with oregano.