Preheat the oven the to 425ºF (220ºC) with the rack adjusted to the upper center position. Line a rimmed sheet pan with a sheet of parchment paper long enough to extend over the sides (this helps prevent the juices from flowing onto the pan, making for quick and easy clean up).
Add the filling ingredients to a medium bowl. Stir until well mixed; set aside.
Prepare the pie crust: add the flour, sugar, and salt to the bowl of a food processor. Pulse to blend. Add the butter. Pulse a few times until it forms a sandy-looking mixture with some pebble-sized pieces.
Add the vinegar, egg, and 2 tablespoons (30 ml) very cold water. Pulse just until it forms a solid ball (may take up to 1 minute).
Divide the dough into 8 equal sized pieces. Gently roll them into balls. Place on a lightly floured surface. Flatten the balls slightly into a circle using the palm of your hand. Dust the tops with a bit of flour, then using a rolling pin, shape into 6-inch (15-cm) circles. Don't fret if they're not perfectly round—making pie should be fun, not stressful.
Spoon an even amount of filling onto one half of each pastry circle, making sure to leave a 1/2-inch (1-cm) border.
Fold the unfilled half of dough over the filling. Press the edges down to seal the pies closed into half moon shapes. Transfer to the prepared baking baking sheet. Use the tips of a fork to crimp the edges closed. Brush the tops with a bit of the egg wash. Sprinkle tops with sugar, if using. Use a fork to prick a few holes in the top so the pies don't explode while baking (the tip of knife works fine, too).
Bake for 20 minutes, then reduce oven temperature to 350ºF (180ºC). Bake for about 20 minutes more, until the pies are golden, and juices begin to bubble slightly. Let pies cool at least 15 minutes before serving. Once cooled completely, pies may be stored in an airtight container for up to 3 days.