Whisk the flour and salt together in a large bowl. Rub in the butter until mixture resembles coarse crumbs. Stir in just enough cold water to bind the dough together. Gather the dough into a ball, wrap in cling film and let it rest in the refrigerator for at least 1 hour.
Preheat the oven to 200 C / Gas 6. Toss the apple slices with the caster sugar, lemon juice and lemon zest.
Roll 2/3 of the dough out on a lightly floured work surface, and line a 23cm tart tin with the pastry. Spread the apples into the pastry case. Roll the remaining pastry and drape over the tart to cover. Pinch the edges together to seal. Using a sharp knife, cut a few vents in the lid for steam to escape.
Whisk together the egg yolk and water, and brush the mixture over the lid.
Bake in the preheated oven until golden, about 45 minutes. Let cool before removing from the tart tin.