Preheat the oven to 220 °C. Rinse the veal, dry it and cut it into thin slices. Season with salt and pepper. Place on 4 large skewers and brush with olive oil.
Peel the onions, cut in half and slice into very thin strips with a sharp kitchen knife.
Place the veal skewers on a wire rack and broil on the middle rack of the oven for 20 minutes. Turn to the other side a couple of times during this process.
Cut deep into the sides of the flatbreads, but do not cut through.
Form a few onion rings, a tiny bit of yogurt sauce and a little nigella into each bread.
Strip the grilled veal slices from the skewers and spread evenly into the bread pockets. Top with the leftover onion rings, yogurt sauce and nigella to taste.
Tip: Use plain or light yogurt as needed!