A butterflied leg or shoulder of lamb is a great way to feed a crowd. Obviously the thicker the lamb, the longer it will take to cook, so when you butterfly the meat, make sure it's no thicker than a steak - you can ask your butcher to do this for you.
Note Dried black olives are available from delicatessens, if they're unavailable, substitute other black olives. Shanklish, a hard cheese coated in za'atar and spices, is available from Middle Eastern grocers. If it's unavailable, substitute feta.