Try afghan brothers restaurant on old airport street or al nasr street; i remember seeing whole camel for parties/catering on their menu so maybe they'll have lamb too; you can call them and ask...i personally love their food and can vouch for the awesomeness of their flavours. However, this will be the arab way of slow cooking the meat with their own recipes so it won't be the lamb or recipe you might expect in our own countries with heavy spices and flavours. Best of luck.