Stir together the flour and salt. Cut in the cold butter with your fingers or a pastry cutter until no large pieces remain.
Add the water. Stir until a dough forms. If the dough is dry, add more water–one tablespoon at a time.
Lightly flour your counter and turn the dough onto the counter. Press into a disk. Wrap tightly and chill for at least 2 hours.
Remove dough from the refrigerator. Use as directed below.
Preheat oven to 375℉.
In a large bowl, stir together peach slices, blueberries, sugar, cornstarch, and vanilla extract.
Roll pie crust out to about a 13-inch round. Place it onto a parchment-lined baking sheet.
Spoon the filling into the center of the pie dough, leaving about 2-inches of dough uncovered.
Fold the outside edge of the dough over the filling. Pleat the dough every inch or so, as needed.
Whisk together the egg and water. Brush the egg over the crust and then sprinkle with sugar. Dot the butter pieces over the filling.
Bake until the filling bubbles and the crust is brown, about 45 minutes.
Let the galette cool for about 25 minutes before serving. Store leftovers in the refrigerator for up to four days.
