Preheat the oven 200 (180 fan)
Line a pie dish with one of the puff pastry sheets and prick the base with a fork
Line the puff pastry with a sheet of parchment paper and fill with baking beans
Blind bake the pastry in the oven for 15 minutes
Once baked, leave it to the side to cool down
Place the chicken, veg and stock into a large pan and bring to the boil
Simmer for 15 minutes and then take it off the heat
In another pan, add the butter and onions and cook until they are soft
Add in the flour and mix with a whisk until it forms a paste
Using a ladle, take the vegetable stock from the chicken and veg mix and add it to the paste. Add a ladle at a time until most of the stock is used
Add the milk to the sauce. Keep whisking until the mixture thickens up.
Once the sauce has thickened, add it to the chicken and vegetables and stir until evenly coated
Transfer the mixture to the puff pastry dish
Brush a little bit of egg around the edges of the puff pastry
Using the 2nd batch of puff pastry, place a lid on top of the pie sealing the edges with pinches
Brush the egg on top of the lid and put a few holes on top (to let hot air escape)
Place the pie back into the oven and bake for a further 22 minutes
Serve this delicious dish with a side of chunky chips (or a salad)