1. Place the chicken and vegetables into a large pot; simmer for at least 1 hour until the meat comes off the bones.
2. Drain, reserving the liquid; cool until lukewarm and remove the meat from the bones. Cut the meat into pieces.
3. Sauté the mushrooms in a non-stick skillet until they give off their liquid; sauté in butter with the chopped onion until lightly browned.
Assembly and cooking
1. Line the baking dish with pastry, letting it hang over the edges.
2. Add the chicken pieces, mushrooms, onion and cream of mushroom soup; cover with a top layer of pastry.
3. Crimp the two crusts together to seal them; place in a preheated 180 °C (350 °F) oven for about 20 minutes or until the pastry is nicely browned.
For a more professional look, brush the top crust with egg yolk mixed with a spoonful of water 10 minutes before the end of the baking time to give your pie a nice golden glaze.