Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper. In a bowl, stir together the sugar, cinnamon and salt.
Place the puff pastry on the parchment paper. Whisk together the egg and water and brush it all over the puff pastry. Sprinkling a few tablespoons of the cinnamon sugar all over the puff pastry. Place the apples on top, in whatever decoration you would like. I used the cuts in the pastry as a guide to layer really thin apple slices. Sprinkle a little more of the cinnamon sugar over the apples.
Bake the pastry for 25 minutes, or until golden and puffed. Remove it from the oven and cut into squares. Serve immediately. Serve with caramel sauce if you'd like!