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Heat a third of the butter in a large saucepan, cook the leeks for 10 mins or until softened, then tip onto a plate and repeat with another third of the butter and the mushrooms. Heat the remaining butter in the same saucepan and, once melted, stir in the flour and cook the sandy paste for 1 min, then stir in the chicken stock a ladleful at a time. Leave on a low heat to simmer for 5 mins, then stir the chicken, leek, mushrooms, cream and mustard into the sauce, add the parsley and season with salt and cracked black pepper. Spoon into a 20cm pie dish and leave to cool.
Heat oven to 190C/170C fan/gas 5. Lay a sheet of filo on a chopping board and brush with olive oil, then place on top of the pie with the corners resting on the sides of the dish, then repeat the process, placing the next sheet slightly off-centre – the idea is to fan the sheets of filo. Brush every layer with oil, and on the last layer sprinkle over salt, thyme leaves and nigella seeds. Press the pastry gently into the edge of the dish. Bake for about 1 hr until the pastry is deep golden, then remove from the oven and serve with buttered cabbage and mashed potato, if you like.