Rub butter into the flour, sugar, and lemon zest until the mixture looks like breadcrumbs.
Stir in the beaten egg with a knife until a dough forms.
Divide the pastry, setting aside one-third for the lid.
Roll out the larger piece of pastry and line a 1.2 litre pie dish.
Chill the pastry-lined dish in the fridge for a few minutes.
Preheat the oven to 200°C and place a baking tray inside.
Mix sugar, cinnamon, cornflour, and apples in a bowl.
Fill the pie dish with the apple mixture and brush the rim with egg.
Roll out the pastry lid, cover the pie, seal the edges, and trim excess.
Crimp the edges, make air holes in the center, and glaze with egg.
Decorate with re-rolled pastry leaves, glaze again, and sprinkle with sugar.
Bake for 45 to 55 minutes until golden-brown and the apples are tender.