Insert the shortcrust pastry into your round pie dish. Feel free to spread the pastry out a little so that the edges stick out of the dish.
Preheat your oven to 220 degrees Celsius, with the rack at the bottom of the oven.
Meanwhile, let the pastry rest in the fridge for half an hour.
Peel the apples and cut them into fairly thin slices.
In a large bowl, mix the sliced apples with the flour, powdered sugar, cinnamon, nutmeg, lemon and a pinch of salt. Do it with your hands, it's more fun and you won't break the apples. You should see a sauce appear after a few seconds.
Pour this mixture into your cooled pie dish.
Cut the butter into small pieces and place them on top of the mixture.
Cover your pie dish with the second shortcrust pastry. You can do this in a grid pattern after cutting strips in the pastry, or simply by placing the pastry on the pie dish and cutting a few slits in the pastry so the pie doesn't explode. Seal the two pastries at the edges by hand or with a fork.
Brush the egg yolk over the top pastry and sprinkle with a tiny bit of brown sugar but be careful, if you put too much you risk burning your pastry... If you're not sure, leave out the brown sugar.
Place in the oven for about 25 minutes to allow the pastry to start to colour.
Turn your oven down to 190 degrees Celsius and continue baking for about 1 hour, until the pastry is hard and the sauce is bubbly.
Eat the pie a little warm, preferably with a scoop of vanilla ice cream.