Prepare the Pie Dough:

1. In a large bowl, mix flour, salt, and sugar.
2. Cut in cold butter using a pastry cutter or fork until it resembles coarse crumbs.
3. Gradually add ice water, 1 tbsp at a time, mixing until the dough comes together.
4. Shape into a disc, wrap in plastic wrap, and chill for at least 30 minutes.

2. Make the Apple Filling:

1. In a bowl, mix apple slices with sugar, brown sugar, cinnamon, nutmeg, lemon juice, and cornstarch.
2. Toss well and let sit for 10 minutes.

3. Roll and Arrange the Dough:

1. Preheat the oven to 375°F (190°C).
2. Roll out the chilled dough into a large circle, about 12 inches in diameter.
3. Place the dough on a parchment-lined baking sheet.

4. Assemble Before Folding:

1. Arrange the apple filling in the center of the dough, leaving about 2 inches of border around the edges.
2. Start folding the dough over the apples in a geometric, origami-like pattern.
3. Ensure the folds overlap slightly for a secure shape.

5. Finish and Bake:

1. Whisk the egg and milk together, then brush over the crust.
2. Sprinkle with coarse sugar for extra crunch.
3. Bake for 35–40 minutes or until the crust is golden brown and the apples are tender.