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In a large pan, melt the butter and cook the leeks, covered, for 10 mins until really soft. Remove from the heat and stir through the cheese sauce, chives, chicken, milk and mustard. Mix well and spoon into small dishes or ramekins.
Un-roll the pastry on a lightly floured surface. Use a saucer to cut circles for the pie tops. Place on top, trim to fit, crimp the edges and brush with egg to glaze. Wrap tightly in cling film and freeze for up to 2 months.
To cook from frozen, heat oven to 220C/200C fan/gas 7. Remove the cling film, cover with foil and put on a baking tray in the middle of the oven. Bake for 15 mins, then reduce the temperature to 190C/170C fan/gas 5 and bake for a further 45 mins, removing the foil for the final 30 mins. Serve with green vegetables.
To cook from fresh, heat oven to 190C/170C fan/gas 5. Cook for 45 mins, until the pastry is golden and puffed, and the filling is piping hot.