Preheat the oven to 160°C/fan 140°C/gas mark 3. Rub the oil over the meat and make incisions in the lamb with a sharp knife. Divide the rosemary leaves and a quarter of the sliced garlic between the holes and push in firmly.
Put the lamb in a large roasting tin and tip in the remaining garlic and the red onions, butternut squash, carrots, celery and leeks. Make up 500ml stock using the stock cube and pour 200ml into the tin. Add the tomatoes, then stir the vegetables until coated. Cover the tin tightly with foil. Roast for 4 hours until the lamb is falling off the bone, the vegetables are tender and the sauce is thick.
Remove the lamb from the tin and rest, covered with foil, for about 10 minutes. Slice and serve with the vegetables and sauce.