In a large bowl, whisk together 1.5 cups all-purpose flour and 0.5 teaspoon salt. Add the cold, cubed butter and cut it into the flour mixture using a pastry blender or your fingertips until it resembles coarse crumbs with some pea-sized pieces of butter remaining.

Gradually add 4-6 tablespoons of ice water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix. Form the dough into a flat disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes, or up to 2 days.

While the dough chills, prepare your apples. In a large bowl, combine the sliced apples with 0.25 cup granulated sugar, 1 teaspoon ground cinnamon, 1 tablespoon lemon juice, and 1 tablespoon all-purpose flour. Toss gently until all the apple slices are evenly coated.

Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper. On a lightly floured surface, roll out the chilled dough into a roughly 12-14 inch circle, about 1/8 inch thick. Carefully transfer the rolled dough to your prepared baking sheet.

Arrange the seasoned apple slices in the center of the dough, leaving a 2-inch border all around. You can layer them in a spiral pattern or simply overlap them. Gently fold the dough border up and over the outer edge of the apples, pleating it as you go to create a rustic, free-form crust.

In a small bowl, whisk the large egg with 1 teaspoon of water to create an egg wash. Brush the exposed crust of the galette generously with the egg wash, then sprinkle the turbinado sugar over the crust for a beautiful sparkle and crunch.

Bake for 30-35 minutes, or until the crust is deeply golden brown and the apples are tender and bubbly. If the crust starts to brown too quickly, you can loosely tent the galette with aluminum foil. Remove from the oven and let it cool on the baking sheet for at least 15-20 minutes before slicing and serving. This allows the filling to set beautifully.