Hard-boil the egg (check the recipe). While cooking, prepare the other ingredients.

Wash, dry, and chop the parsley/chives. In a small dish, combine yogurt, horseradish, and parsley/chives. Season to taste with salt and pepper. Mix thoroughly.

Wash, dry, and halve the tomatoes. Wash, peel, and finely chop the cucumber. Peel and finely chop the onion. Peel the boiled egg and cut it into quarters. Drain and flake the tuna with a fork.

On a plate, evenly spread the washed, dried, and chopped lettuce, onion, tomatoes, cucumber, tuna, egg, and sauce.