Prepare the Dough: In a large bowl, whisk together the flour, 1 tbsp sugar, and salt. Cut in the cold, cubed butter using a pastry blender or your fingers until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

Add Ice Water: Gradually sprinkle in the ice water, 1 tablespoon at a time, mixing gently until the dough just comes together. Do not overmix. Form the dough into a disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes.

Prepare the Filling: In a separate bowl, gently toss the thinly sliced peaches with 1/4 cup sugar, cornstarch, cinnamon, nutmeg, and lemon juice until evenly coated. Set aside.

Assemble the Galette: Preheat oven to 400°F (200°C). On a lightly floured surface, roll the chilled dough into a rough 14-inch circle. Carefully transfer the dough to a parchment-lined baking sheet.

Fill and Fold: Arrange the peach slices in the center of the dough, leaving a 2-inch border clear. Fold the edges of the dough up and over the fruit, overlapping slightly to create the rustic galette shape. Brush the folded crust with the egg wash and sprinkle generously with turbinado sugar.

Bake: Bake for 40 to 45 minutes, or until the crust is deep golden brown and the peach juices are visibly bubbling thickly. If the edges brown too quickly, loosely tent them with foil.

Finish and Serve: Let the galette cool on the baking sheet for 10 minutes. While warm, gently brush the exposed fruit filling with the warmed apricot jam for a beautiful shine. Serve warm or at room temperature, optionally with vanilla ice cream.