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Melt the butter in a heavy pan and add the leeks. Cook gently for 5 minutes, add a splash of water, reduce the heat, cover the pan and sweat for 10 minutes. Add the flour and stir for 1 minute. Take the pan off the heat and gradually add the milk. Return to the heat and bring to the boil, stirring; reduce the heat and simmer for 5 minutes. Season, add the mustard, chicken and ham. Heat through, then add the parsley and nutmeg to taste. Cool.
Preheat the oven to 200C/400F/gas mark 6. Spoon the chicken into an enamel pie dish (measuring about 20x25cm/8x9in). On a floured surface, roll the pastry out to the thickness of a 50 pence piece. Cut off a strip, wet it and place it on the lip of the pie dish. Lay the rest of the pastry on top, pressing down firmly. Trim off the excess. Crimp the edges and use the pastry scraps to decorate. Make 3 cuts in the middle to let the steam escape.
Brush with the egg yolk and bake for 30-40 minutes, or until golden.