The larger the piece of meat, the more difficult to barbecue, particularly without a spit. One cannot be absolute in giving instructions because the temperature on one barbecue will be different from another, depending on the type of wood or charcoal used and the distance of the grill from the fire. The answer in any case is to cook it very slowly.
Brush the leg of lamb with sunflower oil and season all over with salt and pepper.
On a very hot barbecue, sear it quickly on all sides, then remove and put it on a big double layer of foil. Squeeze lemon juice on it, rub it with the smashed garlic and dribble over the olive oil. Sprinkle on the rosemary leaves and wrap loosely in the foil, crimping it closed. It must not be tightly wrapped but have plenty of air around the meat.
Move the grill 20 cm / 8 inches away from the fire and put the package on it. Cook, turning from time to time. Add more wood or coals after 1 hour and again after another hour, cooking for a total of 3 hours, when the meat should still be pink but done all the way through and will be very tender.
© 1998 Alastair Little and Richard Whittington estate. All rights reserved.