Prepare the Crust - In a large mixing bowl, combine all-purpose flour and salt. Cut in the cold unsalted butter using a pastry cutter until the mixture resembles coarse crumbs. Gradually add ice-cold water, mixing until a shaggy dough forms.

Chill the Dough - Shape the dough into a disc, wrap tightly in plastic wrap, and refrigerate for at least 3 hours.

Cook Cranberries - Combine cranberries, granulated sugar, and water in a saucepan. Bring to boil over medium heat and cook until cranberries burst, about 10-15 minutes.

Preheat Oven & Prep Baking Sheet - Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Roll Out Dough - On a floured surface, roll out the chilled dough into a circle about ⅛ inch thick and transfer to the baking sheet.

Prepare Apple Filling - Toss the sliced apple with brown sugar, cinnamon, and cornstarch in a bowl.

Assemble Galette - Spread the cranberry filling in the center of the dough, leaving a 2-inch border, and layer the apple mixture on top. Fold the edges of the dough over the filling.

Apply Egg Wash - Brush the dough edges with the beaten egg and sprinkle sugar crystals on top.

Bake - Place the galette in the oven and bake for 45-55 minutes, until golden brown and bubbling.

Cool - Allow the galette to cool on the baking sheet for 30-60 minutes.