Lay out pastry sheet onto a baking tray lined with a silpat mat. Roll the pastry with a rolling pin to stretch it and remove the fold creases.
Cut strips of about 1/2 inch (1 cm) on both sides of the puff pastry.
Crack the egg into a bowl and whisk to make egg wash.
Put lemon juice into a bowl.
Peel and core apples. Using a hand grater or food processor, grate the apples.
Add apples to lemon juice and mix.
Add apple mixture to the bowl with dry ingredients in it and mix.
Spoon apple mixture onto the center part of the pastry. Even it out. Then bring the strips of pastry over the apple mixture from one side to the other. Continue to make a braid of the puff pastry, crossing the strips over each other.
Brush with egg wash.
Put the Apfelstrudel in the oven until golden brown about 35 minutes.
Allow to cool and dust with icing sugar before serving.