Make the dough: In a large bowl, whisk together the flour, salt, and sugar. Add cold, cubed butter and use a pastry cutter or your fingers to cut it into the flour until you have pea-sized crumbs.
Add liquid: Stir the apple cider vinegar into 6 tablespoons of ice water. Drizzle over the flour mixture, stirring gently with a fork until the dough just comes together. Add an extra tablespoon or two of water if needed.
Chill the dough: Divide the dough in half and shape into discs. Wrap in parchment or plastic wrap and chill for at least 30 minutes.
Preheat the oven: Set your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Roll and cut: On a lightly floured surface, roll out one chilled dough disc to about ⅛ inch thick. Cut into 4-inch rounds or rectangles using a biscuit cutter, cookie cutter, or glass.
Fill the pies: Spoon about 1½ tablespoons of apple pie filling onto half of the dough pieces. If the filling is chunky, give it a quick chop before using.
Seal and crimp: Top with the remaining dough pieces. Press the edges to seal, then crimp with a fork. Transfer to the baking sheet and chill for 10–15 minutes.
Egg wash and vent: Brush the tops with beaten egg and sprinkle with sugar (optional). Cut a small slit or “X” in the top of each hand pie to vent steam.
Bake: Bake for 22–26 minutes or until golden brown and crisp. Let cool slightly before serving warm.