Preheat the oven to 200ºc/180ºc fan and grab a 35x23cm dish
Line the base of the dish with one of the sheets of puff pastry (you may have to roll it out slightly to make it a smidge thinner)
Line the pastry with some parchment paper, and fill with baking beans to cover the base
Bake in the oven for 15 minutes, and then remove and take out the baking beans and parchment paper
While you are baking the pastry, fry the bacon lardons in a large frying pan until they turn crispy, and then remove from the pan
Add the leeks to the frying pan and fry until they have started to soften, and then add the chicken chunks and fry for a few minutes
Return the bacon lardons to the frying pan, and add the dried thyme, bay leaves, chicken stock, double cream and cornflour dissolved in 2tbsp cold water
Stir together and bring the mixture to a rolling boil, until the sauce thickens
Season with salt and pepper, and then pour into the dish onto the pastry base
Top the pie with the second piece of pastry, and seal the edges with your thumbs or a fork. Cut a small cross into the middle to allow air to escape
Brush the pastry with milk (and decorate with extra pieces of pastry if you fancy)
Bake the pie in the oven for 25 minutes, until golden brown
Serve immediately with salad, potatoes or veg!
Notes
You can swap the chicken stock for a different flavoured stock if you fancy
You can swap the chicken and bacon for mushrooms, and extra leeks or other veggies if you want a meat free pie
This pie is best served fresh
The pastry on the base is optional, you can just put it on top
You can top it with filo pastry instead if you want a lighter pie