Combine the blueberries and peaches in a large bowl. Stir together the sugar, cornstarch , lemon juice and salt. Pour it over the fruit and toss well.
Remove one of the crusts from the fridge. (The other is good for another week or so, or it can be frozen.)
Roll one of the pie crusts into a… “rustic”shape – like a big circle with ragged edges, until it is about 1/4 inch thick. Place the dough on a parchment-lined baking sheet.
Layer the fruit in the center leaving a 2-inch+ border of crust. Once the fruit is layered, fold the crust over top of it. Brush the crust with the beaten egg wash.
Sprinkle on the cinnamon sugar mixture and press it into the crust. Sprinkle any of the exposed crust with the sugar.
Bake the galette until the crust is golden and the fruit is syrupy, about 40 to 45 minutes. Let it cool slightly before serving with ice cream and a fresh mint leaf.