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Heat the oven to 180C/160C fan/gas 4. In a flameproof casserole dish that is large enough to fit the lamb, snugly scatter over all the vegetables and herbs, and roughly stir in the tomato purée. Sit the lamb on top, pour over the bottle of wine and season generously. Cover the dish and heat up on top of the stove, then put in the oven and leave undisturbed for 3 hrs. Remove from the oven, leave to cool, then chill overnight.
The next day, remove all the hard fat from around the lamb and discard. Lift the lamb out of the dish, scraping away and keeping any of the jellified juices. Pull apart and shred the meat, discarding any large pieces of fat and the bones. Put the lamb back in the dish, then spoon over the jellied sauce and mix with the vegetables. Set aside.
Heat the oven to 200C/180C fan/gas 6. To make the topping, peel and neatly slice the potatoes by hand into rounds. Place in a pan of cold, salted water, bring to a hard boil and drain straight away. Tip back into the pan, pour over the cream and season. Now it’s up to you how fancy you want to be – you can pile the potatoes over the lamb casually, or you can arrange the potato and sweet potato in interleaving circles. Drizzle over any remaining cream from the pan and dot with butter. Bake for 40 mins-1 hr until the top is golden and the sauce is just starting to bubble.