Preheat the oven to 400 degrees. Line a baking sheet with parchment paper.
On a lightly floured surface, roll the crust into a 13 inch round. Transfer to the prepared baking sheet.
Whisk together 1/3 cup sugar, the cornstarch and lemon zest in a large bowl. Add the peaches and toss to mix well. Gently stir in the raspberries. Spoon the filling over the crust, leaving a two-inch border. Fold the edge of the dough over the filling, pleating it as you go around. Lightly brush the edge of the dough with water and sprinkle the edge and the fruit with the remaining 2 tbsp. sugar.
Bake until the filling is bubbling in the center and the crust is browned, about 35 minutes. Let cool 10 minutes. Slide the galette with the parchment paper onto a wire rack. With a spatula, separate the galette from the parchment; slip out the parchment paper and discard. Let the galette cool completely. Cut into 10 wedges.