Pre-heat the oven to 180°C.
Divide the pastry in two and roll one piece on a lightly floured surface into a round roughly 1cm in thickness. Use it to line the base and sides of a 7 inch ceramic pie dish. Prick the base and chill the lined pie dish and the other piece of pastry.
Melt the butter in a large saucepan over a medium heat. Add the flour and whisk until you have smooth roux, cooking it for 1 minute. Whisk in the milk in a slow, steady stream until you have a smooth, thickened sauce. Add the diced gammon, onion, wholegrain mustard and potato to the sauce and stir well.
Simmer over a low heat for 7-8 minutes. Remove from the heat and season to taste. Allow to cool to one side for 10 minutes.
Meanwhile, roll the other piece of pastry out on a lightly floured surface; this piece will be used as the lid so it should be a round roughly 8 inches in diameter.
Remove the lined pie dish and spoon the filing into it. Wet the rim of the lined piece of pastry with a little water. Drape the pastry lid on top and press onto the moistened rim of pastry, using the tines of a fork to help seal them together. Brush the pastry all over with the beaten egg. Bake for 40-45 minutes until golden brown all over.
Remove from the oven and carefully lift out of the pie dish.
Cut into portions and sit on serving plates. Garnish with the mixed salad leaves and a few walnuts before serving.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.