Sprinkle salt and pepper over the racks of lamb. Cover the lamb with mustard mixture and let lamb come to room temperature, about 20 to 30 minutes. Reserve a small amount of the mustard mixture for basting while on the grill. 1Preheat the grill to medium heat. Clean and oil the grill. 2Place the rack of lamb on grill meat side up, with ribs away from direct heat. Do not place the lamb directly over the heat, in this instance we are actually roasting the lamb on the grill. 1Cook for 14-20 minutes turning and basting occasionally with the mustard mixture.