In a large mixing bowl, combine the all-purpose flour and salt. Cut in the cold, cubed unsalted butter until the mixture resembles coarse crumbs, about 5 minutes. Gradually add ice water, 1 tablespoon at a time, until the dough comes together. Shape into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes.

While the dough chills, preheat your oven to 400°F (200°C). Prepare a baking sheet by lining it with parchment paper.

In a separate bowl, toss the sliced peaches and fresh blackberries with granulated sugar, cornstarch, and lemon juice. Mix until the fruit is evenly coated.

After the dough has chilled, turn it out onto a floured surface. Roll the dough into a 12-inch circle, about 1/8-inch thick. Transfer the rolled dough onto your prepared baking sheet.

Spoon the fruit filling into the center of the dough, leaving a 2-inch border. Fold the edges of the dough over the filling, pleating as you go.

Brush the folded edges with beaten egg and sprinkle coarse sugar evenly over the crust. Bake for 35-40 minutes, until the crust is golden brown and the filling is bubbling.

Once baked, cool for about 10 minutes on the baking sheet. Serve warm, ideally with vanilla ice cream.