For the crust, pulse 2¾ cups of all-purpose flour, 3 tablespoons of granulated sugar, 1 teaspoon of salt, and ¼ teaspoon of baking powder in a food processor.
Pulse in 1 cup plus 2 tablespoons of cold cubed unsalted butter, then 7 tablespoons of ice water, until the dough clumps together. Shape into 2 discs, one about 15% larger, then wrap and refrigerate for at least 45 minutes.
For the filling, toss 2kg of peeled and sliced baking apples in a large pot with 1 tablespoon of lemon juice, 1 cup of packed dark brown sugar, a heaping ¼ teaspoon of salt, and a heaping ¼ teaspoon of ground cinnamon.
Toss in 2 tablespoons of all-purpose flour and 2 tablespoons of cornstarch. Cook over medium heat, stirring often, for 11-13 minutes, until the apples soften and the juices thicken. Spread on a baking sheet and cool completely.
Preheat the oven to 425°F / 220°C with a baking sheet in the lower third. Roll the larger disc into a 13in / 33cm circle, fit it into a 9in / 23cm pie dish, and fill with the cooled apples.
Roll the smaller disc to 11in / 28cm, lay it over the filling, and crimp the edges. Brush with 1 beaten egg, sprinkle with 2 tablespoons of granulated sugar, and cut four 3in / 7.5cm slits in the top.
Bake on the hot baking sheet for 15 minutes. Reduce the oven to 375°F / 190°C and bake for about 45 minutes more, until golden brown. Cool for at least 4 hours before slicing.
