Combine the flour, sugar, and salt in a large mixing bowl. Add the butter and shortening and, using a pastry cutter, two knives, or your fingers, work the butter and shortening into the flour until the mixture resembles coarse crumbs.
Add the water, 1 tablespoon at a time, working it just until the pastry comes together. Form the dough into a disk and wrap in plastic wrap. Refrigerate for at least 1 hour and up to overnight.
When you are ready to bake the galette, position rack in center of oven and preheat the oven to 425?F. Remove the pie dough from the refrigerator and allow to warm up slightly.
On a lightly floured work surface, use a lightly floured rolling pin to roll the pastry to a thickness of about 1/8 inch. Sprinkle a bit more flour if necessary so that the pastry does not stick to the rolling pin or the work surface.
Carefully roll the pastry onto the rolling pin and transfer to a parchment-lined rimmed baking sheet. Refrigerate while you prepare the fruit.
Combine the peaches, berries, 1/2 cup of the sugar, cornstarch, and lemon juice in a large mixing bowl and toss gently to combine.
Remove the pastry from the refrigerator and place the fruit in the center of the crust, leaving a 4-inch border. Gently fold the edges of the crust up and over the fruit so that the pastry forms a sort of bowl. Brush the top edges of the crust with a little water and sprinkle with the remaining tablespoon of sugar.
Bake for 20 minutes, until the crust is set and lightly golden around the edges. Reduce the heat to 375?F and continue to bake until the crust is golden brown and the fruit is bubbly inside, about 20 minutes longer.
Using oven mitts or pot holders, transfer the baking sheet to a wire rack and allow to cool slightly before carefully transferring the galette to a serving plate. (Two metal spatulas work well for this.) Serve warm with vanilla ice cream, cr╦me fraiche, or sweetened whipped cream, if desired.
Yield: 1 fruit galette, serving 4 - 6