Combine all ingredients, except lamb, in a sided container. Add the lamb, turn to coat, and then wrap and refrigerate overnight. Turn occasionally. Preheat the oven to 425 degrees F. Place the lamb in a roasting pan. Spoon over the marinade, roast 20 minutes, and then reduce heat to 325 degrees F. Cook lamb to desired degree of doneness, checking its progress with a meat thermometer. 75-90 minutes should yield medium rare in the middle. Baste with pan juices occasionally during cooking. Rest the lamb 10-15 minutes before thinly slicing.