In a large bowl, whisk together the flour, 1 tablespoon of granulated sugar, and 0.5 teaspoon of salt. Add the very cold, cubed butter and cut it into the flour mixture using a pastry blender or your fingertips until pea-sized crumbs form. You want some larger pieces of butter for flakiness!

Gradually add the ice water, 1 tablespoon at a time, mixing gently until the dough just comes together. Be careful not to overmix! Form the dough into a disc, wrap it tightly in plastic, and refrigerate for at least 30 minutes (or up to 2 days).

While your dough is chilling, prepare the nectarines. In a medium bowl, gently toss the sliced nectarines with 0.25 cup granulated sugar, cornstarch, freshly squeezed lemon juice, and pure vanilla extract. Let this mixture sit while you prepare your workspace.

Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper; this will make cleanup much easier and prevent sticking.

On a lightly floured surface, roll out the chilled dough into a rough 12 to 14-inch circle. Don't worry if it's not perfectly round – rustic is the charm of a galette! Carefully transfer the rolled dough to your prepared baking sheet.

Arrange the nectarine slices in a single layer over the dough, leaving a 2-inch border all around. Fold the dough border up over the fruit, pleating it as you go to create that beautiful, free-form rustic edge.

Brush the folded dough edge with the beaten egg and sprinkle it generously with turbinado sugar for a lovely sparkle and crunch. Bake for 35 to 40 minutes, or until the crust is deeply golden brown and the fruit filling is tender and bubbling.

Let the galette cool on the baking sheet for at least 15-20 minutes before slicing and serving. This is important as it allows the fruit juices to set, preventing a runny mess when you cut into it.