Prepare the Crust: Combine the flour, sugar, salt, and baking powder in the bowl of a food processor fitted with the metal blade. Pulse a few times to combine. Add the pieces of butter and pulse until the mixture is crumbly with lots of pea-size clumps of butter within. Add the water and pulse until the mixture is evenly moistened and very crumbly. Dump the dough crumbles onto a work surface and divide into two piles, with one pile about 15% bigger than the other. Shape into two balls of dough and pat each one into a 5-inch (13-cm) disc. Wrap the discs in plastic wrap and refrigerate for at least 45 minutes or up to 3 days to rest.

Prepare the Filling: In a large heavy-bottomed pot or Dutch oven, mix the apples with lemon juice, brown sugar, salt, cinnamon, flour, and cornstarch. Stir until thoroughly combined and both the flour and cornstarch are dissolved. Place the pot over medium heat and cook, stirring often, until the apples soften and the juices thicken, 11 to 13 minutes. Transfer the apples and their juices to a baking sheet, spreading them out evenly. Allow them to cool completely.

Preheat the Oven: Preheat the oven to 425°F (220°C) and set an oven rack in the lower third position. Place a baking sheet on this rack to preheat as well.

Assemble the Pie: Remove the larger disc of dough from the refrigerator. Lightly dust a clean work surface with flour and place the dough on it; sprinkle a bit of flour over the dough as well. Gently knead the dough with your hands just until it becomes soft and malleable. Roll the dough out into a 13-inch (33-cm) circle. Carefully lift the dough and transfer it to a 9-inch by 1½-inch (23-cm by 3.75-cm) deep pie pan. Gently press the dough into the pan, making sure to ease it into place without stretching. Trim the edges of the dough to extend about ½ inch (1.25 cm) beyond the pie pan's lip. Transfer the cooled apples and their juices into the crust; refrigerate while you prepare the top crust.

Remove the second disc of dough from the refrigerator and roll it out into an 11-inch (28-cm) circle. Drape it over the chilled pie and gently press around the edges to seal. Trim any excess dough, then crimp the edges with a fork or flute them with your fingers. Brush the top with egg wash, cream, or milk, and sprinkle with sugar. Cut four 3-inch (7.5-cm) slits in the top crust to allow steam to escape.

Bake: Place the pie on the preheated baking sheet and bake at 425°F (220°C) for 15 minutes. Then reduce the oven temperature to 375°F (190°C) and continue baking for an additional 45 minutes, or until the crust is golden brown. Let the pie cool completely on a wire rack before slicing, at least 4 hours.