In a mixing bowl, combine all-purpose flour and salt, then cut in the cold, cubed unsalted butter using a pastry cutter until the mixture resembles coarse crumbs. Gradually add ice-cold water, stirring until a dough forms. Shape the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes to set.

Preheat your oven to 400°F (200°C) and prepare a parchment-lined baking sheet.

In a separate bowl, toss sliced peaches with fresh blackberries, granulated sugar, cornstarch, and lemon juice. Let it sit for a few minutes.

Once the dough has chilled, roll it out into a 12-inch circle. Transfer it to your prepared baking sheet.

Spoon the fruit mixture into the center of the rolled-out dough, leaving a 2-inch border. Fold the edges over the fruit, pleating as needed.

Brush the edges with beaten egg and sprinkle coarse sugar on the crust. Bake for 35-40 minutes.

Once baked, cool on the baking sheet for about 10 minutes before slicing and serving.