Trim the excess fat off the lamb leg, then make 2cm-deep crisscross cuts all over it. Salt the flesh and insert the garlic slivers into the cuts at regular intervals.
Mash the garlic to a paste with a pestle and mortar. Mix with the olive oil, lemon juice, oregano, thyme, rosemary and bay, and season with a little salt and plenty of pepper. Place the lamb in a large porcelain dish. Pour the marinade over the lamb and massage into the meat. Leave the lamb for at least 1 hour at room temperature, or longer in the fridge, but no longer than overnight or the meat will become too soft.
Take out the lamb at least an hour before you want to cook it, to give it time to come up to room temperature. Fire up a charcoal barbecue for which you have a lid – aim for a temperature of about 160C – then put the lamb on the grill, cover with the barbecue lid and leave to cook for an hour and a half.
Move the lamb into a roasting tin along with the onions, chicken stock and white wine. Cover the lamb with baking paper and then wrap the tin in at least 2 layers of kitchen foil. Put the roasting tin back on the BBQ and cook for 1 hour.
Unwrap the tin and add the potatoes and some water if required and then rewrap and roast for a further hour.