Step 1: In a large bowl, whisk together the flour, salt, and granulated sugar. Cut in the cold cubed butter using a pastry cutter or your fingers until the mixture resembles coarse meal with some pea-sized lumps of butter remaining.
Step 2: Gradually add ice water, one tablespoon at a time, mixing gently until the dough just starts to come together. Form into a disk, wrap in plastic, and refrigerate for at least 1 hour.
Step 3: Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Step 4: In a medium bowl, toss the sliced nectarines with honey, cornstarch, and lemon juice until evenly coated.
Step 5: On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Transfer it to the prepared baking sheet.
Step 6: Arrange the nectarines in the center of the dough, leaving a 2-inch border around the edges.
Step 7: Fold the edges of the dough over the fruit, pleating as you go to create a rustic border.
Step 8: Brush the dough border with the beaten egg and sprinkle with coarse sanding sugar.
Step 9: Bake for 35-40 minutes, or until the crust is deep golden brown and the fruit juices are bubbling.
Step 10: Allow the galette to cool for at least 15 minutes before slicing to allow the juices to set.