To make the pastry, rub the flour, sugar, and butter together between your fingertips until it resembles breadcrumbs. (You can also whizz in a blender). Make sure your hands are cold when handling pastry at all stages – run them under cold water before you start.
Add a small amount of water and some of the beaten egg yolk and bring the mixture together to form a dough. Work the pastry as little as possible.
When it has come together in a ball, wrap it in cling flim and pop it in the fridge for 30 mins.
While the pastry is chilling, Preheat the oven to 200oC. Grease a circular baking tin or ceramic dish.
Peel and roughly chop the apples. Put into a pan of boiling water and stew for a few minutes until soft, but not mushy. Drain the apples and sprinkle over brown sugar and lemon zest, stirring in. Set the apples to one side and allow to cool.
Roll out the pastry on a floured surface until it is fairly thin. Lift carefully into the baking tin and press around the bottom edge. Cut away the excess pastry. Pierce the pastry base with a fork (or cover with baking beans) and blind bake for 15-20 mins.
While the pastry case is baking, roll out the remaining pastry to a large enough size to make a lid.
Fill the pastry case with the apples and top with the pastry lid. Glaze with the beaten egg yolk and bake for about 30 minutes until pastry is golden.
While the pie is cooking, make the custard. Put the egg yolks, cornflour, sugar, and vanilla extract into a pan and whisk with a balloon whisk until combined. Add the milk and cream slowly, whisking together all the time.
Put the pan over a low heat and stir continuously until it thickens. Remove from heat and put into a jug. Stand the jug in a pan of hot water to keep it warm
Remove the pie from the oven and serve while hot with the custard. It can also be frozen for another day.
Notes
To store your pie, put it into an airtight container and keep it refrigerated for a maximum of 2 days. Alternatively, freeze it for up to 3 months.