The day before serving, mix all marinade ingredients. Place in a pint jar and shake well.
Remove excess fat from lamb without breaking the parchment-like fell, leaving a little fat in order to enhance flavor. Using a small brush, spread surface of meat well with marinade. Place lamb in a large pan, cover, refrigerate overnight or for 24 hours. Baste occasionally with marinade.
The following day, remove meat from refrigerator at least 40 minutes before cooking time, in order to bring to room temperature. Heat coals. Either lay leg of lamb on grill or place on a spit. Turn meat over every 15 minutes. Cook to desired doneness.
Transfer meat to a pan and let rest for 10 minutes before carving. (Resting meat makes it firm and easier to carve.)