Ingredients:-
2 x 320g rolls of ready-rolled puff pastry
3 chicken breasts - cut into small chunks
3 carrots - peeled and chopped into small chunks
1 cup diced celery
3 medium sized potatoes - peeled and chopped into small chunks
good pinch of salt and pepper
2 sprigs of thyme or 1 tsp dried thyme
2 ½ cups chicken stock
3 ½ Tbsp (50g) butter
1 large onion - peeled and finely chopped
6 Tbsp (50g) flour
1 ¼ cups (300ml) milk 
Juice of 1 lemon
1 egg - slightly whisked with a fork
METHOD:-
Preheat the oven to 200°C.
Line a 10 x 8-inch pie dish with one roll of the puff pastry.
Prick the base several times with a fork and cut off any large overhanging bits of pastry.
Line the pastry with baking parchment so that it completely covers the pastry, then fill with baking beans (or dried beans) and place in the oven to 'blind bake' for 10 minutes.
Take out of the oven and put to one side.
Meanwhile, place the chicken, carrots, celery, potatoes, salt, pepper, thyme and chicken stock to a large saucepan.
Bring to the boil and allow to simmer for 15 minutes and then turn off the heat.
When the chicken and stock is nearly cooked, take another large sauce pan and add in the butter and chopped onion. Heat on medium and cook the onion for about 5-6 minutes until soft.
Add in the flour and mix it into the onions and butter using a whisk. It should form into a creamy paste (or roux). Allow the roux to cook for a minute whilst stirring with your whisk.
Then add in a ladle of the stock from the chicken broth (try to get mostly stock - not the chicken/veggies). Stir the broth into the mixture using the whisk. Repeat this until you have ladled out most of the stock from the chicken pan (leaving the chicken and veggies behind).
Now pour in the milk. Heat through whilst stirring with the whisk. The sauce should thicken as it is heating.
Once the sauce is almost at boiling (don't let it boil) add in the chicken and veggies.
Bring back to almost boiling and add in half the lemon juice. Give it a stir and taste. Add the rest of the lemon juice and a more seasoning if you think it needs it.
Remove the baking beans and baking parchment from the pie and fill the pie up with the chicken mixture.
Brush the edges of the pastry with egg wash, then top the pie with the remaining piece of pastry.
Pinch the edges to seal, then brush the top with more egg wash and pierce a couple of small holes in the top of the pie with a sharp knife.
Place back in the oven (still at 200c/400f) and cook for 20-25 minutes until golden brown.
Serve immediately with crusty rolls.