Preheat oven to 220°c.
Divide puffed pastry into 3 long rectangles and put them onto a baking pan that’s been lined with greaseproof paper.
Score the sides of each piece with a knife leaving a 1 cm border.
Put into the oven 8-10 minutes until they are puffed and just starting to brown.
While waiting, core and sliced the apples into very thin slices.
Separate the egg yolk and the white into 2 different dishes.
When the pastry base is slightly cooled, brush the base egg white and arrange the apples in a neat row over each sheet.
Sprinkle the brown sugar and cinnamon over the apples generously.
Dice the butter and use 3 tsp each for each piece.
Beat the egg yolk slightly and brush on the outside rim/border of the pastry shell.
Place back into the oven for another 10-25 mins until golden brown.
Use a Sieve to dust icing sugar over strudel. Best served with a big scoop of vanilla ice cream.