Combine apples, 3/4 cup sugar, raisins, walnuts, and cinnamon in a large bowl for the filling.
Dust a flat work surface lightly with flour. Unroll puff pastry and sprinkle lightly with flour. Roll out slightly and mark into 3 equal sections. Spoon filling into the central section then fold over the section on the left and brush with egg. Fold the right section on top, just like a letter. Make shallow diagonal cuts in the top layer of the apple strudel. Brush with egg and sprinkle with 1 tablespoon sugar.
Bake in the preheated oven until apple studel is puffed up and golden, 25 to 30 minutes.