Make the dough: Cut your butter into small chunks and place in the freezer for 30 minutes. Whisk together the flour, salt, and sugar and place in the freezer for 30 minutes.
In a large bowl, combine your cold butter into the flour mixture using a pastry blender or food processor until the butter is the size of peas. Add the water and apple cider vinegar and incorporate into the flour until a dough forms. If the mixture is too crumbly and does not come together when pressed, add more cold water, one tablespoon at a time, until the desired consistency is reached. Cover with plastic wrap and chill in the fridge for at least one hour.
Once the dough is chilled, preheat the oven to 425 degrees.
Make the filling: In a bowl, gently combine the sliced peaches, brown sugar, salt, cinnamon, and flour. Add the blackberries at the end.
On a floured surface roll out the dough into a flat disc to the diameter of approximately 11 inches and an 1/8 of an inch thick, fold over and place on a parchment-lined baking pan. Lay the dough flat and arrange the fruit inside in concentric circles or whatever pattern you prefer leaving an approximately 2 inch border. Fold over the edges onto the fruit and brush the dough with egg wash.
Place in the oven for 35-45 minutes, rotating the pan halfway. Allow to cool before serving.