Cut the apple into 8 even pieces, then chop into smaller pieces. Soak in salted water, and transfer to a pot with the sugar, lemon juice and 150 ml water. Simmer for about 15 minutes over medium heat.
When the water has evaporated, turn down the heat to low. Add the cinnamon power and maple syrup, and simmer some more. When the mixture has cooled, the filling is ready. Divide it into 6 equal portions.
Bring the frozen puff pastry to room temperature, and cut the sheets into 12 portions so you have a top and bottom piece for each pie. Make cuts on 6 sheets, which will be used to cover the filling.
Spoon the filling on the sheets without the cuts, leaving the edges of the pie sheets, and don't put in too much filling during and cover with a sheet with cuts. Press down the edges with a fork.
Line a baking tray with parchment paper. Brush the glaze on the pies, and bake in the oven preheated to 210℃ for about 10 minutes.
When the surface starts to brown, turn down the temperature to 170℃, bake for another 15 minutes, then they're done.