Prepare the Dough: In a large bowl, mix flour, sugar, and salt. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Gradually add ice water, one tablespoon at a time, until the dough comes together. Wrap in plastic wrap and refrigerate for 30 minutes.
Preheat Oven: Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
Prepare the Filling: In a medium bowl, combine sliced peaches, blackberries, brown sugar, cornstarch, and vanilla extract. Toss gently to coat the fruit evenly.
Roll Out the Dough: On a lightly floured surface, roll out the dough into a rough 12-inch circle. Transfer it to the prepared baking sheet.
Assemble the Tart: Arrange the fruit mixture in the center of the dough, leaving a 2-inch border around the edges. Fold the edges of the dough over the fruit, pleating as needed to create a rustic look.
Add Final Touches: Brush the crust with beaten egg and sprinkle with coarse sugar for a golden, crunchy finish.
Bake: Bake for 40 minutes or until the crust is golden and the fruit is bubbly. Allow to cool slightly before serving.