Cook the egg until hard-boiled (check the recipe). While the egg is cooking, start preparing the other ingredients.

Wash the lettuce, dry it, and cut it into strips. Wash the cherry tomatoes, dry them, and cut them in half. Wash the chives, dry them, and chop finely. Drain the tuna and flake it with a fork. Peel the cooked egg and cut it into quarters.

Arrange the lettuce, cherry tomatoes, tuna, and egg evenly on a plate. Spread the mustard on top and sprinkle with chives. Season to taste with pepper.