Center a rack in the oven and preheat to 350 degrees F.
To make the fruit filling, combine all ingredients in a large bowl and toss gently, making sure that the cornstarch is coating the fruit. (This will prevent too much juice from leaking out while the galette bakes.)
Remove the dough from the refrigerator and place on a sheet of parchment paper. Starting at the center of the dough, roll out, forming a 14-15-inch wide circle. Place the dough with parchment paper on a baking sheet. Add the fruit mixture to the center of the dough, leaving a 2-inch border. Fold the border over the filling, overlapping where necessary and pressing gently to adhere the folds.
Lightly whisk the egg white and milk in a small bowl. Brush dough with egg wash, and sprinkle with sugar. Bake for 1 hour, or until crust is golden brown and the filling is bubbly.
Transfer the baking sheet to a wire rack to cool for at least 20 minutes. (Cutting it too quickly will cause the dough to crack). It's best served at room temperature or slightly warm.
Optional garnishes include vanilla ice cream, a dollop of creme fraiche, or freshly whipped cream.
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