Prepare the puff pastry. Once it has softened a bit after taking it out of the freezer, but before it gets too warm, open one sheet and roll it out in both directions so that it becomes slightly larger.
Then, cut it into six rectangles and place each on a plate between parchment paper sheets. Refrigerate while you prepare the apple filling. If using one apple, you’ll have two rectangles left over).
Peel and slice the apple. Try to cut even apple slices that are not too thin.
Place in a pot and cover with the sugars, cinnamon, and pinch of salt, and cook over low heat until the sugar becomes syrupy. Do not add water.
Cook the apples for about 10 minutes or until they soften a bit but still hold their shape.
Once the apples are ready, remove them from the heat. Prepare a baking sheet with parchment paper and then add two thin slices of butter for each rectangular tarte.
Layer the apples on top of each of the butter slices. Arrange them using tongs or your fingers.
Place a piece of puff pastry over each of the apple rectangles.
Pinch with a fork on all four sides of each of the four sides. Cut slits and pierce with a fork.
Brush with an egg wash and bake for 15 minutes in a 350º oven. Watch closely.
Remove from the oven and let sit for a minute or two, then using a flat spatula, carefully flip them over as you move them to a cooling rack.
The apples reveal themselves just like a tarte tatin.
Serve warm or room temperature with a little vanilla ice cream.