Preheat the oven to 160°C, fan 140°C, gas 3. Rub the oil over the meat and make incisions all over 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 vegetables. 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 mins. Slice and serve with the vegetables, sauce and the potato and celeriac mash.
To freeze, allow the lamb to cool before removing from the bone and transferring to a freezer-proof container, along with the sauce and veg. Freeze for up to 1 month. Defrost overnight in the fridge.