To make the dough, place the flour, salt, and sugar in a bowl, cut in the cold butter with a pastry cutter. When the mixture looks like breadcrumbs or oatmeal flakes, add the water and mix with the fingers of one hand to bring into a ball. Knead lightly to get the mixture to come together. Place in wax paper in the fridge to chill for at least 20 minutes. To make the filling, in a large bowl whisk 2/3 cup of the sugar with the cornstarch. Gently toss in the peaches. Let sit for 15 minutes, tossing occasionally. Stir in the blueberries. Roll out the dough on a lightly floured surface to about 14 inches around. Or, cut 2, 6-inch rough circles. Transfer the rounds to a parchment-lined or non-stick baking sheet. Mound the filling into the center of the large or smaller rounds, brush the edges with water, and fold the sides up around the fruit leaving a large opening in the middle. Brush water on the top edges, sprinkle with the remaining sugar. Bake the galettes on the lower rack of a preheated 400 degree oven for about 35 minutes. Let cool for 5 minutes and remove from baking sheet. Serves 6.