Preheat your oven to 400°F and line a baking sheet with parchment paper.
Roll out the puff pastry sheets on a lightly floured surface. Cut it in half, then into thirds, for a total of 6 rectangles, about 6×3 inches each. You should get a total of 12 pieces of puff pastry
Spoon about ⅓ cup of apple filling onto the center of each rectangle, leaving a ½-inch border around the edges
Place a second puff pastry rectangle on top of the filling and press gently around the filling to remove air. Using a fork, press the edges of the top and bottom sheets together to seal the pies. Make 3 small parallel incisions into each pie to allow steam to escape.
Beat the egg in a small bowl and brush the tops of the pies with the egg wash, then sprinkle each pie with a little granulated sugar.
Transfer the pies to the cookie sheet and bake for 16–20 minutes, or until the pastry is puffed and golden brown.
Let the pies cool slightly before serving. Enjoy them warm or at room temperature.