For the pastry, combine the flour, salt, sugar and lemon zest and rub in the cold butter until the mix resembles breadcrumbs. Beat the egg with ice-cold water and add to the flour mixture to form a dough. Divide the dough into two halves. Refrigerate for at least 30 minutes. The key to working with this pastry is keeping everything cold. The lemon zest also adds an extra layer of flavour to a fairly simple and well-known recipe.
Take one half of the pastry dough, roll it out, and place it in the pie dish. Return the pie dish to the fridge whilst you prepare the filling.
Cut the apples into thin slices and add lemon juice to prevent discolouration. Mix the sugar, cinnamon, nutmeg, cornflour, and roughly-chopped walnuts (optional) and combine with the sliced apples. Add the apple mixture to the pie dish.
Roll out the other half of the pastry. You can just cover the top of the pie with the pastry as a closed lid or cut the pastry into strips for a lattice top as pictured above.
Glaze the top with a beaten egg. Sprinkle with sugar. Bake for 45–55 minutes or until golden brown.