To prepare the pastry, combine the flour, salt, and butter pieces in a large bowl. (Make sure the butter pieces are 1⁄4 inch or smaller.) Add 6 tablespoons of the water and mix with a fork just until the dough comes together. Add 1 to 2 more tablespoons water if you need to, but stop before the dough gets too wet. Turn the mixture out onto a lightly floured work surface. It should just hold together when you squeeze it in your hand. With a lightly floured rolling pin, shape the dough into a 6×12-inch rectangle with a short side nearest you.
Fold the dough into thirds like a letter. (Fold the bottom third up and then the top third down over the bottom third.) Use a bench scraper to help lift and fold the dough if necessary. At this point, the dough will be very rough and shaggy. As you roll and fold the dough it will come together. Rotate the dough so that the folded edge is to the left. Repeat rolling and folding two more times, lightly flouring the surface as necessary. Wrap the dough tightly with plastic wrap and refrigerate for at least 1 hour. Repeat the entire process one more time for a grand total of six turns. Wrap the dough well and refrigerate until cold, about 4 hours or up to 2 days. Alternatively, freeze the dough, well wrapped, for up to one month.
Line a baking sheet with parchment paper. Divide the dough into two equal pieces. Working with one piece at a time, roll the dough on a lightly floured surface into an 8×15-inch rectangle. Cut the dough into twelve 21⁄2×4-inch rectangles. Put the rectangles on the prepared sheet and lay another piece of parchment on top. Repeat with the remaining dough. Cover the sheet with plastic wrap and refrigerate the dough on the sheet until ready to use. Don’t worry if the rectangles aren't perfect. Once the dough has rested you can roll and trim each piece to neaten them up if you’d like.
To prepare the filling, in a small saucepan, whisk together the sugar, cornstarch, nutmeg, cinnamon, and salt. Stir in the blackberries and peach and cook over medium heat until the fruit has broken down and the mixture has thickened, 6 to 8 minutes. Let cool completely.
Preheat the oven to 400° Line two baking sheets with parchment paper.
In a small bowl, make an egg wash by whisking together the egg yolk and the 2 teaspoons water. Lay out 6 dough rectangles on one of the prepared sheets. Dollop about 2 tablespoons of filling in the center of each rectangle. Brush the edges with some of the egg wash and top with another dough rectangle, pressing the edges with a fork to seal. With a paring knife, make a little slit in each pie to vent steam. Chill the assembled pies for 20 minutes.
Repeat with the remaining dough and filling on the other prepared baking sheet. Brush the tops of the chilled pies with egg wash. Bake until puffed and golden brown, 20 to 25 minutes. Repeat with the remaining pies. Let the pies cool slightly on a rack.
To prepare the glaze, whisk together the confectioners’ sugar, cream cheese, and vanilla in a small bowl. Whisk in enough milk to make a thick glaze. Drizzle over the warm pies. Serve warm or at room temperature. These are really best the day they’re made, but you can store leftovers in an airtight container at room temperature for up to 2 days. Glazed pies don’t reheat well, but unglazed pies can be reheated in the oven on low heat.