Line a pie dish with the short crust pastry and blind bake for 20 minutes at 200 degrees celsius.
To make the béchamel sauce, place the milk with the onion, mace peppercorns and bay leaf in a saucepan and slowly bring to simmering point. Remove from the heat and leave for the flavour to infuse for 8 – 10 minutes.
Meanwhile, to make the filling, melt 50g of butter in a frying pan and sear the chicken. Add the onion, garlic and mushrooms and fry until soft. Add a splash of white wine to deglaze the pan and throw in the parsley.
In a separate pan, melt 20g of butter, remove from the heat and stir in the flour, cooking over a heat for 1 minute. Remove from the heat and gradually add the strained milk to the mixture. Return the sauce to the heat, add the mustard, and stir continuously until boiling. Simmer, stirring well, for 2 minutes and season.
Put the chicken and mushroom mixture into the pie dish and then pour a layer of béchamel over it. Cover with the puff pastry, pinching around the edges, and splash with egg wash. Put into a preheated oven at 200 degrees celsius for 30 minutes and then serve with mashed potato.