In a large mixing bowl, whisk together the flour, 1 tbsp granulated sugar, and salt. Add the cold, cubed butter and cut it into the flour mixture using a pastry blender or your fingertips until the mixture resembles coarse meal with some pea-sized pieces of butter remaining.

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

While the dough chills, prepare your filling. In a separate large bowl, gently combine the sliced nectarines, fresh blueberries, 1/3 cup granulated sugar, cornstarch, and lemon juice. Toss carefully to coat the fruit without mashing it.

Preheat your oven to 400°F (200°C). On a lightly floured surface or between two sheets of parchment paper, roll the chilled dough into a rough 12-14 inch circle, about 1/8 inch thick. Carefully transfer the dough circle to a parchment-lined baking sheet.

Arrange the fruit filling in the center of the dough, leaving a 2-inch border all around. Carefully fold the dough border up and over the fruit, pleating it as you go to form a rustic edge. Don't worry about making it perfect; the charm is in its imperfections!

Brush the folded crust with the beaten egg and sprinkle generously with turbinado sugar. This will give it a lovely golden shine and a delicious crunch.

Bake for 35-45 minutes, or until the crust is deeply golden brown and the fruit filling is bubbly and tender. If the crust starts to brown too quickly, you can loosely tent it with aluminum foil.

Let the galette cool on the baking sheet for at least 15-20 minutes before slicing and serving. This allows the filling to set properly. Enjoy warm or at room temperature.