Melt the butter in a saucepan and add the onion. Cook gently for a few minutes until beginning to soften. Add the mushrooms and cook while stirring occasionally for 2 minutes.
Stir in the flour and cook for 1 minute. Remove from the heat and slowly stir in the stock, followed by the milk. Return to the heat and cook while stirring until the sauce thickens. Stir in the mustard and season to taste. Allow to cool stirring occasionally to prevent a skin forming.
Preheat the oven to 210℃ (190℃ fan)/425°F /gas mark 7. Divide the pastry into two and roll out each piece into a large circle. Use one piece to line a 20cm/8in deep pie plate.
Stir the cooked chicken into the cooled mushroom sauce and spoon into the centre of the pie.
Moisten the edge of the pastry with water and place the second circle of pastry on top. Trim the edges and press the edges together to seal.
Make a slit in the centre of the pie and use the pastry trimmings to decorate the top. Secure in place with a little beaten egg. Brush the whole of the pastry top with beaten egg and bake for 20 minutes, then reduce the heat to 190℃ (170℃ fan)/375°F /gas mark 5 and cook for a further 20 minutes.
Nutrition information is approximate and is meant as a guideline only. (It does not include seasoning with salt and pepper.)