Preheat the oven to 150°C fan forced (300°F)
Season the lamb with salt on all sides
Place the garlic in the base of a large heavy-based roasting pan, then top with the lamb
Place the onions, rosemary, thyme and bay leaves in around the lamb, season with some black pepper, then pour over the stock
Place a piece of baking paper over the top of the lamb, then cover the pan tightly with foil
Roast the lamb for 4 hours
Increase the oven temperature to 200°C fan forced (390°F)
Remove the foil from the pan and roast for a further 20 minutes, until the lamb skin begins to brown and crispen
Transfer the lamb to a serving dish and keep warm
Remove all the hard herbs from the pan, then transfer the pan sauce, with the garlic and onions, to a jug and allow the fat to settle to the top
Skim off the fat, then puree with a stick blender until smooth
Pour the sauce over the lamb and pull the meat apart from the bone with carving forks to serve
