First of all, preheat your oven to 200 degrees Celsius and place your puff pastry sheets on a flat surface to defrost to room temperature.
Meanwhile, cut your apples in half from top to bottom and thinly slice them, flat side down, slicing off any tough core areas.
Take your greaseproof paper (parchment paper) and place it onto your oven tray or baking sheet.
Now place your defrosted pastry sheets onto the tray and prick with a fork, leaving a border around the edge.
Nıow brush your pastries with the egg wash, paying extra attention to the edges.
Sprinkle your cinnamon over the pastries.
Now layer your apple slices along your pastry sheets, allowing them to overlap and leaving a gap around the perimeter of your puff pastry dough.
Sprinkle the apples with your almond flakes and now place your tray into the centre of the preheated oven.
Bake for 15-20 minutes until the apple slices have released their juices and you have golden brown flaky crusts around the edge.
Remove from the oven and sieve the caster sugar over your apple tarts.
Serve immediately or leave to cool and store in an airtight container in the fridge for up to three days.