Peel and finely chop the onion. Knead minced meat, egg yolk, diced onion and breadcrumbs. Season with salt and pepper. Form into 6 small balls and fry them in hot oil all around for about 5 minutes. Dice the feta cheese and cut the olives into slices. Wash, clean and slice the tomatoes.
Clean, wash and dry the salad. Cut the baguette rolls deep, break them apart a little and spread a thin layer of mustard over them. Line with lettuce leaves. Distribute 1 meatball, cheese cubes, tomato and olive slices each. For transport, pin them down with skewers if necessary