Preheat the oven to 200c/400f. Unroll the 3 pastry rolls. Slice one of the pieces of pastry in half length-ways. Attach a cut piece to one of the whole pieces of pastry - so you have a larger piece of pastry that will totally cover the base of the pie dish. Repeat with the other piece of pastry - so you have two large pastry sheets altogether.
Line a 26cm (10.2") diameter pie dish with the prepared one-and-a-half rolls of the puff pastry. Trim the edges, leaving a 1-2cm overhang, as the pastry will shrink back slightly.
Prick the base of the pastry several times with a fork. Line the pastry with baking parchment so that it completely covers the pastry. I find it easier to scrunch up the parchment first, and then un-scrunch it, so it fits into the shape of the pie dish better.
Fill the pie with baking beans (or dried beans or uncooked rice) and place in the oven to 'blind bake' for 10 minutes. This helps to prevent the pie having a soggy bottom.
Take out of the oven and put to one side. When cool enough to handle, remove the baking beans and parchment paper.
Now start on the filling. Add the oil to the pan and fry the chicken pieces in the oil for 5-6 minutes until sealed. Remove from the pan.
Add the butter and onions to the pan and turn down the heat to medium-high. Cook for 3 minutes, until the onions start to soften.
Add the salt, pepper, thyme and flour to the pan and stir until the onions are coated in the flour.
Add the stock, a splash at a time, whilst stirring with a balloon whisk, until all of the stock is used. Adding it in a splash at a time, whilst stirring, will ensure you have not lumps. Add the milk and continue to stir until the sauce starts to bubble and thickens.
Add the mushrooms, chicken, bacon and lemon juice and stir. Turn off the heat (don’t worry, the mushrooms will cook through in the oven).
Fill the pie base 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.
Trim off any excess pastry (you can using the trimming sot make little pie decorations such as leaves if you like), 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.
Can I make it ahead?
Yes - make and cook the whole pie ahead of time, but cook for about 5 minutes less than the stated time. Then cool, cover and refrigerate the pie. When you want to reheat the pie, take it out of the refrigerator about 30 minutes before you put it in the oven (to take off a little of the chill). Preheat the oven to 180C/350F. Cover the pie in foil, and bake for about 25 minutes - until hot throughout (stick a knife or skewer in the middle to check it's piping hot). Then remove the foil and cook for a further 5 minutes to crisp up the pastry..
Can I freeze it?
Yes - make and cook the whole pie ahead of time, but cook for about 5 minutes less than the stated time. Then cool, cover and freeze the pie. Defrost thoroughly in the refrigerator, then as above, take it out of the refrigerator about 30 minutes before you put it in the oven (to take off a little of the chill). Preheat the oven to 180C/350F. Cover the pie in foil, and bake for about 25 minutes - until hot throughout (stick a knife or skewer in the middle to check it's piping hot). Then remove the foil and cook for a further 5 minutes to crisp up the pastry..
Why do I need to blind bake the pie?
The pie isn't in the oven long enough to completely cook the puff pastry on the base of the pie before the top starts to burn. Blind baking the pastry will ensure the pie doesn't have a soggy bottom..
You can buy baking beans, but I use a mixture of dried beans and rice, which I think works just as well. You can't use the beans and rice in a recipe once you've used them as baking beans, so instead I wait for them to cool, then store them in a jar. They can then be re-used as baking beans many times..
I don't like mushrooms
You can leave out the mushrooms and add more bacon. Or you can replace the mushrooms with leeks (sliced raw leeks are fine) or some sauteed carrots or chunks of par-cooked potato..