Remove the lamb from the refrigerator at least 45 minutes before cooking to allow it to come to room temperature. Cold meat placed directly on a hot grill will cook unevenly.
Set up your grill for indirect cooking. For a charcoal grill, bank the coals to one side and place the lamb on the opposite side, away from direct heat. For a gas grill, light one side only and place the lamb on the unlit side. The lamb should cook primarily through indirect heat with the fat rendering slowly rather than flaring in direct contact with flames.
Place the lamb on the grill. Baste frequently with any remaining marinade, applying a fresh coat every 15 to 20 minutes throughout the cooking process. The basting builds up layers of caramelised cumin and garlic on the surface of the lamb that define the finished flavour.
Cook until the internal temperature reaches your desired doneness. For a medium result, the internal temperature should reach 145°F. For a more traditional well-done result appropriate to the ancient preparation, cook to 165°F. Total cooking time will be approximately 1.5 to 2 hours depending on the size of the leg and the temperature of your grill.
Rest the lamb for at least 15 minutes before serving. The resting period allows the internal juices to redistribute through the meat and produces a significantly better result than cutting immediately after cooking.