Preheat oven 220°C, gas mark 8. Scatter the onions, garlic and half the rosemary over the bottom of a deep-sided roasting tin. Place the lamb on top, slash the skin and rub with the oil then season. Place the remaining rosemary on top, pour over the stock and wine, then cover tightly with foil. Place in the hot oven and immediately reduce the heat down to 150°C, gas mark 2. Leave to slowly cook for 4 hours, or until the meat is falling off the bone, removing the foil halfway through.