In a food processor, combine the flour with the sugar, salt and butter and process for about 5 seconds. Sprinkle the ice water, mixed with lemon juice or vinegar, over the flour mixture and process until the pastry just begins to come together, about 10 seconds; you should still be able to see small pieces of butter in it.

Transfer the pastry to a work surface, gather it together and pat into a disk. Wrap the pastry in plastic or wax paper and refrigerate for at least 30 minutes.

Mix the flour, sugar, cinnamon and salt. Cut the butter in with a pastry blender or your fingers until the mixture resembles coarse crumbs. Set aside.

Peel, halve and core the apples and slice them crosswise ¼ inch thick. Toss with the sugar, cornstarch, and cinnamon.

Preheat the oven to 400°. On a lightly floured piece of parchment paper, roll out the pastry to a 14-inch circle. It will overhang the short sides of the paper. Transfer to a large rimmed baking sheet. Spread the apples over the pastry to within 2 inches of the edge.

Dot with the pieces of butter and sprinkle with the streusel. Fold the pastry edge up and over the apples to create a 2 inch border. Chill the galette for 15 minutes while you preheat the oven to 400°F.

Whisk an egg with one teaspoon of water. Brush the folded pastry dough with egg wash and sprinkle lightly with granulated sugar.

Bake the galette at 400°F for 45 minutes to an hour, or until the pastry is nicely browned and crisp and all of the apples are tender.

Transfer the pan to a rack and let the galette cool. Serve warm or at room temperature.