In a large bowl, whisk together the flour, granulated sugar, and salt.

Add the cold, cubed butter and use a pastry cutter or your fingers to work the butter into the flour mixture until it resembles coarse crumbs.

Gradually add the ice water, 1 tablespoon at a time, and mix until the dough comes together. Do not overmix.

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

In a large bowl, combine the sliced peaches, granulated sugar, cornstarch, lemon juice, ground cinnamon, and ground nutmeg (if using). Toss gently to coat the peaches evenly.

Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.

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

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

Fold the edges of the dough over the peaches, pleating as needed to create a rustic look. The center will be open.

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

Bake in the preheated oven for 40-45 minutes, or until the crust is golden brown and the filling is bubbly.

Allow the galette to cool for at least 10 minutes before slicing and serving.

Serve the peach galette warm or at room temperature. Optionally, top with a scoop of vanilla ice cream or a dollop of whipped cream.