Pierce the surface of the meat with a fork. Mix all of the ingredients together, except the butter, and rub into the meat thoroughly. The Afghans keep the meat at room temperature for about 12 hours, but I prefer to cover it and keep it in the refrigerator overnight.
Place the lamb in a shallow roasting pan, dot the surface with the butter, then place it in the oven for about 15 minutes at 350 degrees F. Reduce the oven heat to 300 degrees F. and cook for three hours, or longer if needed. Baste with the juices in the pan and serve hot with a rice side dish.