Heat oven to 200C/fan 180C/gas 6. Peel and chop the onion as finely as possible. Melt the butter in the pan over a medium heat. Add the chopped onion and leave to cook for 5 mins, stirring occasionally. Finely slice the mushrooms and add them to the pan with the onions.
When the onions and mushrooms are almost cooked, add the flour to the pan and stir to make a thick paste called a roux. Crumble the stock cube in and stir well. Continuously stir the roux over a low heat for 2 mins to cook out the flour. This stops the sauce from having a starchy taste.
Slowly add the warm milk to the pan, stirring all the time. Once all the milk has been added, stir in the stock, season with salt, pepper, nutmeg and mustard powder. Add the bay leaf and bring to the boil, stirring all the time to stop the sauce becoming lumpy.
When the sauce has thickened, place on a very low heat. Chop the cooked chicken into even-sized pieces and add to the sauce. Add your veg and pour into the pie dish.
Put the pastry on a clean, floured surface. Make sure you also dust the rolling pin with flour. Roll out the pastry, trying not to make any tears. Measure the pastry against the pie dish and once the pastry is slightly larger than the top, lift it carefully over the dish. It might be helpful to use the rolling pin to help you move the pastry. Trim off the pastry hanging over the edge of the dish. Press the outside edge of the pastry with a fork or your fingers to make a nice pattern.
Brush the top of the pie with beaten egg or milk. Make a small hole in the centre of the pastry top to allow the steam to escape. You can use any leftover pastry to decorate the top of the pie with pastry shapes if you like. Place the pie in the oven for 25 mins. It is ready when golden brown.