In a food processor, combine flour, sugar, and salt, pulsing to mix. Cut in cold butter until the mixture resembles sandy crumbs. Gradually add ice water and apple cider vinegar until a dough forms. Wrap in plastic wrap and chill for 2 hours or in the freezer for 1 hour.

Preheat your oven to 400°F (200°C).

In a large bowl, combine diced peaches and rhubarb with brown sugar, cornstarch, and a pinch of salt. Stir until fruits are evenly coated. Set aside.

Roll out the chilled dough into a ¼-inch thick circle on a floured surface. Transfer to a parchment-lined baking sheet.

Spoon filling onto the center of the dough, leaving a 1-2 inch border. Fold the dough over the fruit, creating a pleated edge.

Whisk together the egg and a splash of water for the egg wash. Brush over the crust and sprinkle with sanding sugar if desired.

Bake for about 45 minutes, until the crust is golden brown and the filling is bubbly.

Cool for 15-20 minutes before slicing and serving.