Rub the butter into the flour, sugar and salt until it resembles breadcrumbs. Mix in the egg yolk and enough cold water to bring together into a dough. Wrap and chill for at least 30 minutes.
Toss the sliced apples with sugar, cinnamon, lemon zest and flour until evenly coated.
Roll out just over half the pastry and line a 23cm pie dish. Pile in the filling, dot with butter, top with the remaining pastry, trim and crimp the edges, and cut steam vents.
Brush with beaten egg, sprinkle with sugar, and bake at 200°C (180°C fan) for 20 minutes, then reduce to 180°C (160°C fan) for a further 25-30 minutes until golden and bubbling.