Combine first 8 ingredients in a large heavy-duty zip-top plastic bag; add lamb. Seal and chill 8 hours, turning occasionally.
Prepare fire by piling charcoal or lava rocks on each side of grill, leaving center empty. Place a drip pan between coals. Coat food rack with cooking spray, and place on grill.
Remove lamb from marinade, discarding marinade; place lamb on food rack over drip pan.
Grill, covered with grill lid, 2 hours or until a meat thermometer inserted into thickest portion registers 145°. Remove from heat; cover and let stand 20 minutes or until meat thermometer registers 150° (medium-rare). Garnish, if desired.