Preheat the oven to 425 F and place a rack in the center of the oven.
Line a baking sheet, with a sheet of parchment paper.
Make the egg wash by whisking the egg with two tablespoons of water and a sprinkle of salt in a small bowl.
Briefly roll out the sheets of puff pastry just so they are even and smooth. Use flour as needed to keep it from sticking.
Cut both sheets of puff pastry into 6 even rectangles.
On the bottom set of six, evenly distribute the apple filling into the centers of the rectangles.
On the bottom set of six, evenly distribute the apple filling into the centers of the rectangles.
Top the apples with the mashed blackberries, use a fork to scoop them up trying to leave as much of the moisture behind.
Brush the edges of the 6 bottom rectangles with the egg wash. Lay the top set of rectangles over the top of the bottom set. You have to slightly stretch each rectangle to fit it over the pile of filling.
Press the edges of the two sheets of pastry together with your fingers first. Then use a fork to crimp the edges together so they are fully sealed.
Use the same fork to prick the top of each pastry rectangle three times so that steam can escape.
Brush the top of the pastry with more egg wash and sprinkle with turbinado or demerara sugar.
Bake the pastry in the preheated oven for 15 minutes, then rotate and bake for another 15-20 minutes until deeply golden brown.
Once baked, remove the sheet from the oven and place on a wire rack to cool for 10-15 minutes minimum.
If you plan to fill them with custard, let them cool completely first!
To fill with custard, use a pairing knife to cut a small slit in one short end of each pastry. Use a piping bag to pipe 1/6 of the custard into each pastry.