Whisk the flour, sugar, and salt together in a medium bowl.

Using a pastry cutter or 2 forks, cut in the butter until the mixture resembles coarse pea-sized crumbs.

Add the water and stir until the flour is moistened. Add 1–2 more Tablespoons of water if the dough seems dry.

Turn the dough out onto a lightly floured work surface and, using your hands, work the dough into a ball. Flatten it into a thick disc.

Wrap the dough disc in plastic wrap or parchment paper and refrigerate it for at least 1 hour, and up to 3 days.

Mix the apples, brown sugar, flour, lemon juice, cinnamon, and nutmeg together in a large bowl. Cover tightly and let sit until the dough is ready.

Line a large baking sheet with parchment paper or a silicone baking mat.

On a lightly floured surface, roll the dough into a 12-inch circle.

Transfer dough to the prepared baking sheet.

Arrange the apples into the center of the dough, leaving a 2–3-inch border all around them.

Gently fold the edges of the dough over the filling, overlapping the dough as necessary. Press gently to seal the edges.

Brush the crust edges generously with egg wash and sprinkle the crust with coarse sugar, if desired.

Refrigerate the shaped galette for at least 15–20 minutes as the oven preheats.

Preheat oven to 400°F (204°C).

Bake until the filling is bubbly and the crust is golden brown, about 35–36 minutes.

Remove from the oven and allow to cool on the baking sheet for 10 minutes before slicing and serving.