To make the sumac dressing, place the vinegar, olive oil and sumac in a bowl and whisk to combine. Set aside.
Place the lamb, sumac, olive oil, salt and pepper in a bowl and toss to coat. Heat a non-stick frying pan over medium heat. Cook the lamb for 2 minutes each side for medium or until cooked to your liking. Arrange the lettuce, cucumber, feta, mint and lamb on plates and drizzle with the sumac dressing to serve. Serves 4.