Preheat an oven to 200 degrees, bake and defrost 2 sheets of puff pastry. 
In a large deep casserole pot or saucepan, heat the element to a high temperature. 
Add 1 tablespoon of butter. 
When the butter is sizzling and at temperature, add the leek, chicken thighs and potatoes and cook on high for a few minutes. 
Reduce the temperature to a medium simmer and pour in the stock liquid and add the fresh thyme. Allow the chicken and potatoes to simmer for 15-20 minutes in the liquid or until the chicken is cooked through to an opaque inner colour. 
In a large frying pan add the other tablespoon of butter over a medium temperature. 
Pan fry the onion for a few minutes until glossy and translucent. 
Add the flour or corn flour and stir to coat the onion. 
Now pour in the milk whilst you are constantly stirring the ingredients in the pan to help the flour blend in with the liquid. 
Drain the casserole pot liquid stock into a jug and pour the jug into the frying pan to help thin out the sauce. Allow this liquid to simmer for around 5 minutes until the liquid is thick and creamy. 
Now transfer the creamy sauce back to the chicken, potatoes and leek in the pot and add in the smoked bacon pieces. 
Season well with salt and pepper, extra thyme if you would like and gently fold through to combine. 
In a large pie dish, pour in the chicken, leek and smoked bacon pie filling. 
Place 2 sheets of store bought puff pastry over the top of the pie and cut off any excess. Brush the pastry top lightly with the egg wash and bake the pie in the oven for 30 minutes or until the pastry has puffed up and is a beautiful golden colour. 
Remove the pie from the oven and allow it to cool for 10 minutes before slicing and serving. The pie will keep in the fridge in an airtight container for up to 3 days. When reheating, ensure that you heat the pie and the insides of the pie until piping hot. We do not recommend reheating the pie more than once due to food safety.