Place the red onion slices in a bowl with vinegar and allow them to pickle while you prepare the platter.
For the dressing:
Place all the ingredients in a screw-top jar. Season. Seal and shake well to combine. Set aside to develop the flavours.
For the lamb:
Preheat a barbecue grill or chargrill pan on high. Rub the lamb steaks with olive oi land season, then cook for 3 minutes on each side for medium-rare or until cooked to your liking. Remove to a plate, cover with tin foil and rest for 5minutes. Then slice the lamb.
Arrange the sliced lamb, lettuce, Feta, pickled onions, cucumber, tomatoes and tzatziki on a serving platter. Drizzle with lemon and garlic dressing and serve with warm flatbreads.