In a large mixing bowl, combine all-purpose flour and salt. Cut in cold, cubed butter until the mixture resembles coarse crumbs. Gradually add ice water, mixing gently until the dough begins to form. Shape into a disk, wrap in plastic wrap, and refrigerate for 30 minutes.

Preheat your oven to 400°F (200°C). Toss sliced peaches and blackberries with granulated sugar, cornstarch, and lemon juice in another bowl. Set aside to allow the flavors to meld.

After 30 minutes of chilling, roll the dough out into a 12-inch circle on a lightly floured surface. Transfer the dough to a parchment-lined baking sheet.

Spoon the fruit mixture into the center of the rolled dough, leaving a 2-inch border. Carefully fold the edges over the filling, pleating as necessary.

Brush the edges with beaten egg and sprinkle coarse sugar over the crust. Place in the oven.

Bake for 35-40 minutes until the crust is golden brown and the filling begins to bubble.

Allow to cool for 10 minutes before slicing. Serve warm, optionally with ice cream.