Preheat the oven to 130°C/Gas Mark ½.
Mix the potatoes, onions and thyme leaves together in a bowl and season. Roughly layer the potatoes and onions in a roasting tray and put the lamb on top, skin side up. Use a knife to pierce the lamb all over, then put a clove of garlic into each hole. Pour over the chicken stock.
Put the baking tray in the oven and roast the lamb for 4–5 hours, until the lamb is tender and the potatoes are cooked through. Remove the tray, cover it with foil and leave the lamb to rest for 20 minutes before carving.