To make the dough, start by mixing the flour, sugar and salt together in a bowl. Add the butter and use a pastry cutter or a fork to cut the butter into the flour. Do this for about two minutes, until the butter is broken into pea sized pieces.
Whisk together the egg and two tablespoons of cold water, then pour it into the flour mixture. Use a fork to stir the mixture until it comes together, adding one to two additional tablespoons of water until it forms a ball. Do not knead the dough or mix any further. Wrap the ball in plastic and refrigerate for at least 30 minutes prior to rolling out.
Bring a large pot of water up to a boil, prepare an ice bath, and score an X on the bottom of each peach. Drop the peaches into the boiling water for about 30 seconds, or until the skin begins to peel up from the cut ends. Remove from the water and immediately submerge into the ice bath. Once cool enough to handle, the skins should easily peel off. cut the peaches in half, remove the core, slice into ¾ inch pieces and place in a bowl.
Preheat the oven to 425 degrees. Add the blueberries to the bowl along with the salt, sugar, cinnamon and flour, then mix to combine. Roll out the dough to about a 12" circle and transfer to a parchment lined baking sheet. Pour the fruit mixture into the center leaving a 2-inch border around the outside. Gather and fold the dough up around the sides to enclose the filling.
Dot the butter around the top of the filling. Brush the exposed dough with the egg wash, then sprinkle the raw sugar on top. Bake for about 25-30 minutes, or until the edges are golden brown and the filling is bubbling. Serve warm, ideally with whipped cream or vanilla ice cream.