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Heat the oil in a large pan over a medium-low heat and cook the onion and leek, stirring for 10-15 mins until softened. Push the veg to one side of the pan, turn up the heat to medium, and add the lamb mince. Fry for 10-15 mins, breaking up the mince with a spoon, until all the liquid has evaporated and the lamb is browned in places. Cut the preserved lemon into wedges, then remove the pith and finely chop the peel.
Add the lemon peel, garlic, sweet potato, cumin, harissa and tomato purée to the pan, season and sizzle for 1-2 mins. Stir in 200ml water and scrape any browned bits from the bottom of the pan. Put on a lid so it’s partially covered, then cook for 10 mins until the sweet potatoes are starting to soften but still holding their shape, and the mixture is fairly dry. Remove from the heat and leave to cool completely. Once cool, will keep chilled for up to two days or frozen for two months.
For the pastry, put the butter, lard, flour and 2 tsp salt in a food processor and blitz until the mix resembles breadcrumbs. With the motor running, slowly drizzle in 6 tbsp cold water and pulse until the dough starts to clump together, adding another 1-2 tbsp water if the dough looks very dry. Tip the dough out onto a work surface and knead briefly to bring it together. Alternatively, tip the ingredients into a bowl and rub together using your fingertips before mixing in the water. Shape the dough into a disc, then wrap and chill for 30 mins.
Cut the dough in half so it’s easier to work with, then roll out one half on a lightly floured surface to the thickness of a £1 coin. Use a 10-12cm cutter to stamp out circles of pastry – you should get about 20 once you’ve re-rolled trimmings and the second half of dough.
Working with one pastry circle at a time, brush a little beaten egg around the edge of the pastry and spoon 1 heaped tbsp of the lamb mixture into the centre. Fold the pastry over and crimp the edges together to seal, or press with the tines of a fork – don’t worry if the pastry cracks a little, as you can just pinch it back together. Arrange the pasties on a baking tray lined with baking parchment. Once all the pasties are assembled, brush with more beaten egg and sprinkle with the nigella seeds. Chill for at least 30 mins. Can now be frozen for up to two months, as long as the mince mixture was not previously frozen.
Heat the oven to 200C/180C fan/gas 6 and bake for 35 mins from chilled or 45 mins from frozen until the pastry is golden. Leave to cool for 10 mins before serving warm.