Preheat the oven to 375ºF. Line a baking sheet with a parchment paper and set aside.
To make the apple filling, peel, core, and chop apples into 1/2-inch cubes. You will need total 3 cups of chopped apples. Place them in a bowl and add 2 tablespoon of white sugar, flour, and cinnamon. Toss well and set aside for 15 minutes so the apple can release its juice.
Meanwhile, combine nuts, brown sugar, the remaining 2 tablespoon of white sugar, and breadcrumbs in another small bowl. Stir well and set aside.
To assemble the strudel, roll out the thawed puff pastry on a lightly floured work surface to stretch out a little, about 10x12-inch. Sprinkle half of nut mixture in the center of the pastry leaving 1-inch from both sides. Spoon half of apple mixture on top of nut mixture.
Lift the front side of pastry and fold over the filling. Lift the opposite side of pastry and fold over. Pinch the sides down to seal. Carefully transfer the strudel to a prepared baking sheet, placing the seam-side down. Tuck both ends under. Repeat the same on the other pastry sheet.
Brush the strudels with egg wash and make slits for the steam to escape. Bake the strudel in a preheated 375˚F oven for 40-45 minutes until golden brown. Let the pastry cool for 10 minutes.
Dust with powdered sugar and serve warm with vanilla ice cream and/or whipped cream.