Make the pastry dough: Whisk together the dry ingredients in a medium bowl. With a pastry cutter, cut the butter into the flour mixture until the largest pieces are about the size of a pea. Drizzle the ¼ cup of water over the mixture, and using a wooden spoon gently mix together. If it looks like it is too dry, add up to another ¼ cup of water. Do not overmix. Once everything is incorporated, gather into a ball and cut in half. Wrap each half in a piece of plastic wrap, flatten into a disk, and chill for at least an hour.
Make the filling: In a mixing bowl, whisk together the sugar and cornstarch. Add the fruit, and stir gently with a wooden spoon to coat. Set aside.
Assembly: Preheat the oven to 375 degrees. On a work surface with a piece of floured parchment over it, roll out one of the dough halves to about a 12 inch round. Spoon half of the blackberry/nectarine filling into the middle of the dough, leaving a 2 inch border. Fold the edges over to the middle in whatever arrangement you like (I tried to do about 5 or 6 folds). Brush with egg wash, and press sliced almonds into the folded crust. Repeat process with second dough and remaining filling. Sprinkle with coarse sugar and bake for about 55 minutes until crust is browned and filling bubbles. Allow to cool on a wire rack before serving.