For the salad, pre-cook barley and lentils to pack directions, drain well and refrigerate until required.
Combine sumac, oil and seasonings. Rub all over lamb rumps and set aside covered for 20 minutes.
Pre-heat a hooded barbecue to 200°C, place lamb on a baking or foil tray and place on barbecue plate. Turn off the burner directly under where the lamb is sitting. Pull lid closed and cook lamb for 25 minutes for medium. Allow to sit covered for 15 minutes before slicing.
Combine barley and lentils with the remaining salad ingredients. Toss well.
Serve sliced lamb with salad and accompany with yoghurt, sumac, mint and lemon wedges.