Peel and grate the apples.
Mix in the brown sugar and cinnamon.
Spread the apple filling on the dough in a thin layer. Leave about 2 cm on all the edges.
Roll up the dough. First flip the clean edges on the shorter sides onto the apple filling. Then start rolling up along the long edge. Once you get almost all the way to the other end flip the long clean edge onto your rolled dough and stick it on.
Line a baking tray with baking paper. Carefully lay the strudel onto the tray with the connected long edge facing down.
Whisk the egg and do a light egg wash all over the strudel.
Bake in the oven at 210 °C for about 25 mins (or follow the baking instructions on the package of your puff pastry dough).
