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

Add Ice Water: Sprinkle the ice water, one tablespoon at a time, mixing lightly until the dough just comes together. Form into a disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes.

Prepare the Filling: While the dough chills, gently toss the blueberries and sliced nectarines in a medium bowl with the sugar, cornstarch, lemon juice, and vanilla extract 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. Transfer the dough to a baking sheet lined with parchment paper.

Fill and Fold: Arrange the fruit mixture in the center of the dough, leaving a 2-inch border all around. Dot the fruit with the small pieces of butter. Gently fold the edges of the dough up and over the outer edge of the fruit filling, overlapping slightly to create a rustic border.

Bake: Brush the folded crust with the egg wash and sprinkle generously with coarse sugar. Bake for 35-40 minutes, or until the crust is golden brown and the filling is bubbly. If the edges brown too quickly, cover loosely with foil.

Cool and Serve: Let the galette cool on the baking sheet for at least 20 minutes before slicing and serving warm. This dessert pairs wonderfully with vanilla ice cream.