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Put the tomatoes, chillies, oregano, cumin and garlic in a food processor with one-third of the wine, add 1 tsp salt and blitz until smooth.
Put the lamb in the slow cooker pot (we used a 6.5-litre model). Pour over the chilli mix, then add the rest of the wine with the chocolate, cinnamon stick and whole potatoes. Cover and cook on low for 7 hrs until the lamb is really tender and the potatoes are still firm.
To serve, remove the bones and skin from the lamb and pull the meat apart into large chunks. Chop the potatoes into chunks and skim the fat from the cooking juices. Serve the lamb, potato and juices in bowls with the slaw on top.