Make the pie crust: a. In a large mixing bowl, combine the flour, sugar, and salt. b. Add the cold, cubed butter and, using a pastry cutter or your fingers, cut the butter into the flour mixture until it resembles coarse meal with pea-sized pieces of butter. c. Gradually add the ice-cold water, 1 tablespoon at a time, mixing until the dough comes together. You may not need all the water. The dough should not be sticky but should hold together when pinched. d. Divide the dough in half and shape each half into a disc. Wrap each disc in plastic wrap and refrigerate for at least 1 hour or up to 2 days.
Preheat the oven to 425°F (220°C).
Make the apple filling: a. In a large mixing bowl, combine the sliced apples, sugar, flour, lemon juice, cinnamon, nutmeg, and salt. Toss until the apples are evenly coated.
Assemble the pie: a. On a lightly floured surface, roll out one of the dough discs to a 12-inch (30cm) circle. Carefully transfer the dough to a 9-inch (23cm) pie dish, pressing it gently into the bottom and sides. Leave any excess dough hanging over the edges. b. Spoon the apple filling into the pie crust, arranging the apples evenly and creating a slight mound in the center. c. Roll out the second dough disc to a 12-inch (30cm) circle. Place it over the apple filling, aligning it with the edges of the bottom crust. Trim and crimp the edges of the pie crust together, sealing the filling inside. d. Use a sharp knife to make several slits in the top crust to allow steam to escape during baking.
Prepare the egg wash by whisking together the egg and milk or cream. Brush the top crust with the egg wash.
Bake the pie in the preheated oven for 20 minutes at 425°F (220°C). Then, reduce the oven temperature to 375°F (190°C) and bake for an additional 30-35 minutes, or until the crust is golden brown and the filling is bubbly. If the edges of the crust start to brown too quickly, cover them with aluminum foil.
Remove the pie from the oven and let it cool on a wire rack for at least 2 hours before serving. This allows the filling to set.