In a mixing bowl, combine the flour and salt, then add the cold, cubed butter. Using your fingertips or a pastry cutter, work the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces.
Gradually add ice-cold water, one tablespoon at a time, mixing gently with a fork until the dough begins to come together. Stop adding water once the dough is just cohesive without being sticky.
Shape the dough into a flat disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes to relax the gluten and keep it cold.
While the dough chills, prepare the peaches by slicing them thinly and tossing with lemon juice, sugar, and optional spices. Set aside to macerate slightly.
Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. Remove the dough from the fridge and let it sit at room temperature for a few minutes.
Lightly flour a clean surface and roll the dough into a rough 12-inch circle, about 1/8 inch thick. Don’t worry about perfect edges; rustic is beautiful here.
Transfer the rolled dough onto the prepared baking sheet. Gently fold the edges inward, leaving a 2-inch border that you can pleat or fold over slightly.
Spread the sliced peaches evenly over the center of the dough, leaving the border free to fold up around the filling. Drizzle any accumulated juices over the fruit.
Brush the exposed crust with the beaten egg or milk mixture for a glossy, golden finish. If using spices, sprinkle a little over the peaches now.
Bake the galette in the preheated oven for about 40-45 minutes, or until the crust is deep golden brown and crisp. Keep an eye on it to prevent burning, especially at the edges.
Remove the galette from the oven and let it cool slightly for 10 minutes. The crust will be crunchy, and the filling will be bubbling and caramelized at the edges. Slice and serve warm or at room temperature, enjoying the rustic charm and juicy fruit.
