Cook the egg hard-boiled (check the recipe).

Peel the cooked egg and chop it into cubes. Wash, dry, and finely chop the chives or parsley.

In a bowl, place the egg, yogurt, mustard, olive oil, and chives/parsley. Season to taste with salt and pepper. Mix thoroughly.

Cut the roll in half lengthwise. Spread the egg paste evenly on the inner sides of the roll.