Step 1: In a large bowl, whisk together the flour, 1 tablespoon sugar, and salt. Add the cold cubed butter and use a pastry cutter to work the butter into the flour until it resembles coarse crumbs.
Step 2: Gradually add ice water, one tablespoon at a time, mixing until the dough just begins to hold together. Form into a disk, wrap in plastic, and refrigerate for at least 30 minutes.
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, gently toss the blackberries with 1/3 cup sugar, cornstarch, and lemon zest until evenly coated.
Step 5: On a lightly floured surface, roll out the chilled dough into a rough 12-inch circle. Transfer the dough to the prepared baking sheet.
Step 6: Spoon the blackberry mixture into the center of the dough, leaving a 2-inch border around the edges.
Step 7: Fold the edges of the dough over the berries, pleating as you go to create a rustic border.
Step 8: Brush the edges of the crust with the beaten egg and sprinkle with coarse sanding sugar.
Step 9: Bake for 30-35 minutes, or until the crust is golden brown and the berry juices are bubbling.
Step 10: Allow the galette to cool on the baking sheet for at least 15 minutes before slicing and serving.