Preheat oven to 125°C (260°F). Trim the excess fat from the lamb and make a dozen or more cuts deep into the meat with the tip of the knife. Combine the butter, garlic, spices and olive oil and spread the mixture quite thickly over the entire leg of lamb, working some of the butter into the incisions made with the knife. Place the leg of lamb into a roasting pan and cover with foil, sealing the edges tightly. Roast the lamb, basting hourly and resealing the foil each time, for seven to eight hours, or until the juices run clear and the meat is tender enough to pinch off the bone.

Note: Small legs (under 2 kilograms / 4.5 pounds) may finish cooking in six hours; large legs may take closer to nine hours. Remove the foil and increase the oven temperature to 240°C (460°F). Brown the lamb, basting frequently, for 15 to 30 minutes, or until the meat is well colored. Transfer the lamb to a platter and allow it to rest for 10 minutes before serving. If desired, the juices can be poured over and around the lamb.

Serve dishes of salt and cumin on the side for dipping.