Heat the slow cooker on Low. Meanwhile, heat the oil in a frying pan and brown the duck legs in batches until golden. When all the legs are done place in the slow cooker. While you’re browning the duck, put the onion, garlic, chilli, ginger, spices and preserved lemon in a food processor and blitz to a chunky paste. Tip the paste into the hot pan and cook for 5 mins to soften. Meanwhile, add half the dates to the processor with 200ml water, and blitz until smooth. Cut the remaining dates into chunky pieces. Tip the onion paste and all the dates into the slow cooker, add the passata, season, stir, cover with the lid and cook for 5 hrs (or up to 7).