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

Form the Dough: Gradually add the ice water, 1 tablespoon at a time, mixing until the dough just comes together. Do not overmix. Form the dough into a flat disk, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes.

Prepare the Filling: While the dough chills, gently toss the sliced peaches with 1/4 cup sugar, cornstarch, lemon juice, vanilla extract, and cinnamon in a medium bowl. Set aside.

Assemble the Galette: Preheat your oven to 400°F (200°C). On a lightly floured surface, roll the chilled dough out into a rough 12-inch circle. Carefully transfer the dough to a parchment-lined baking sheet. Arrange the peach filling in the center, leaving a 2-inch border clear around the edges.

Fold and Bake: Gently fold the edges of the dough up and over the filling, overlapping slightly to create a rustic border. Brush the exposed crust with the whisked egg wash and sprinkle generously with turbinado sugar.

Bake the galette for 40 to 45 minutes, or until the crust is deep golden brown and the peach juices are bubbling thickly. Let cool on the baking sheet for at least 15 minutes before slicing and serving warm.