How to Make It
In a food processor, pulse the butter with the flour and salt until it resembles coarse meal. Add the water and pulse just until a dough forms. Divide the dough into 6 pieces and flatten each into a disk. Wrap in plastic and refrigerate for 30 minutes.
Preheat the oven to 375°. Spray 2 baking sheets with cooking spray. In a medium bowl, toss the peaches with 1/2 cup of the granulated sugar, the cornstarch and cinnamon; add a pinch of salt. Fold in the berries.
On a lightly floured work surface, roll out each disk of dough to a 6-inch round about 1/8 inch thick. Transfer 3 to each baking sheet. Spread the fruit on the rounds and fold 1-inch of dough over the fruit to form a rim. Top the fruit with a piece of butter and refrigerate for 30 minutes.
Brush the crusts with the beaten egg and sprinkle with the remaining 3 tablespoons of granulated sugar. Bake the croustades for about 50 minutes, or until the crusts are golden, rotating the baking sheets halfway through baking. Transfer to racks to cool for 20 minutes. Dust with confectioners' sugar and serve.