Melt the butter in a large pan, then add the leeks and cook, 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.

Unroll the pastry on a lightly floured surface. Use a saucer to cut circles for the tops, put them on top, trim to fit, crimp the edges and glaze with egg. Will keep, covered and frozen, for up to two months.

Heat the oven to 190C/170C fan/gas 5 and bake for 45 mins, or, to cook from frozen, heat oven to 220C/200C fan/gas 7. Cover with foil and put on a baking tray. Bake for 15 mins. Reduce the oven to 190C/170C fan/gas 5 and bake for 45 mins, removing the foil for the final 30 mins. Serve with green veg, if you like.