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

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

In a medium saucepan, combine cranberries, granulated sugar, and a splash of water. Cook over medium heat until cranberries burst and mixture thickens, about 10-15 minutes.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Roll out the chilled dough into a circle about ⅛ inch thick and transfer it to the prepared baking sheet. Chill again for about 15 minutes.

In a bowl, toss sliced apples with brown sugar, cinnamon, and cornstarch until evenly coated.

Spread cooled cranberry filling in the center of dough, leaving a 2-inch border. Layer apple mixture on top and fold edges over filling.

Brush the dough edges with beaten egg and sprinkle with sugar crystals if desired.

Bake for 45-55 minutes until crust is golden and filling bubbles. Cover edges with foil if browning too quickly.

Allow galette to cool on baking sheet for 30-60 minutes, then serve warm with a scoop of vanilla ice cream or whipped cream.