In a large bowl, combine 1 ½ cups of all-purpose flour, 1/4 teaspoon salt, and 1/4 teaspoon granulated sugar. Mix well.

Add the chilled, diced unsalted butter to the flour mixture. Using a pastry cutter or your fingers, mix until the mixture resembles coarse crumbs.

Gradually add 6 to 8 tablespoons of ice water, one tablespoon at a time, mixing until the dough comes together. Do not overwork the dough.

Shape the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 1 hour.

Preheat your oven to 375°F (190°C).

In a large mixing bowl, combine the sliced apples, brown sugar, lemon juice, ground cinnamon, and 1 tablespoon of all-purpose flour. Toss until the apples are evenly coated.

On a floured surface, roll out the chilled dough into a 12-inch circle. Transfer the dough to a parchment-lined baking sheet.

Arrange the apple mixture in the center of the dough, leaving a 2-inch border around the edges.

Fold the edges of the dough over the apples, pleating it as needed to create a rustic look.

Brush the edges of the dough with the beaten egg and sprinkle coarse sugar over the crust.

Bake in the preheated oven for 30-35 minutes, or until the apples are tender and the crust is golden brown.

Remove from the oven and let cool slightly before serving.