Preheat the oven to 375°F / 190°C / Gas Mark 5.
Remove all meat from the chicken carcase and cut into bite-sized pieces.
Put carcase into a pan of salted water and boil for 30 minutes.
Reserve 250ml of this chicken stock for this recipe. (The remainder can be used to make soup).
Make a white sauce by melting the butter in a saucepan and adding the cornflour. Next whisk in the milk and chicken stock, bring to the boil and allow to thicken.
Place the chicken, onion and mushrooms into a greased pie dish and cover with the puff pastry.
Bake in the oven for about 30 minutes (the pastry should have risen and be golden brown).
