Combine the flour, sugar, cornstarch and salt in the bowl of a food processor. Pulse a few times. Add the cold butter, and pulse until the butter is the size of peas, or slightly smaller. This can also be done using a pastry blender. Pro-Tip: It's important not to pulse the butter too much. Keeping it in the size of peas or a little smaller allows for a flaky crust.
Add the vinegar and 2 tablespoons of ice water, and pulse until the dough begins to come together. Add a little more ice water if needed.
Dump the dough and remaining flour onto a workspace, such as a counter. Using the palm of your hand, smear the dough away from you in a single motion, over the loose flour that was left in the bowl of the food processor. Fold the dough back over itself, and repeat 2 more times. The flour should all be incorporated into the dough. This smearing action is called fraisage, and creates layers of butter, which creates a flaky crust.
Pat the dough into a round disk shape, wrap in wax paper and chill in the refrigerator while you prep the apples.
Preheat the oven to 400˚F and adjust your racks so you can bake the galette in the middle of the oven.
Remove the dough from the refrigerator, and sandwich the dough between 2 sheets of wax paper. Roll out the pie crust to about 13 - 14 inches in diameter. Don't worry if it's not a perfect circle - galettes are rustic by design. Peel off the wax paper and reapply at intervals to allow the dough to move more freely between the wax paper layers when rolled. I do this every time I flip the dough over.
Line a baking sheet with parchment paper. Peel back one layer of wax paper, and transfer the dough to the baking sheet. Peel back the other layer of wax paper. Place the Apple Galette Filling in the center of the dough, and spread gently with your fingertips, leaving a 3-inch border of dough all around. Fold the dough up over the Apple Filling, in pleats as necessary.
Bake for 50 - 60 minutes. The galette crust should be lightly browned and the apples tender.
Remove from the oven, and leave the galette on the baking sheet for 10 minutes before moving it to a cooling rack. There will be some cranberry juice on the bottom of the baking sheet. When ready to transfer the galette, tilt the baking sheet up at an angle, and gently pull on the parchment paper to slide the galette onto the cooling rack. It's important to pull it in one clean motion so it doesn't get hung up on the edge of the baking pan. Using a small pastry brush, baste the top of the galette with the Galette Glaze. The apples, when baked can look a little dry, and caramelized on some edges. The Glaze makes the top of the galette glisten.