Heat the oven to 375 degrees F. Beat the egg and water in a small bowl with a fork or whisk. Stir the sugar, flour and cinnamon in a medium bowl. Add the apples and raisins and toss to coat.
Unfold the pastry sheet on a lightly floured surface. Roll the pastry sheet into a 16 x 12-inch rectangle. With the short side facing you, spoon the apple mixture onto the bottom half of the pastry sheet to within 1 inch of the edge. Roll up like a jelly roll. Place seam-side down onto a baking sheet. Tuck the ends under to seal. Brush the pastry with the egg mixture. Cut several slits in the top of the pastry.
Bake for 35 minutes or until the strudel is golden brown. Let the strudel cool on the baking sheet on a wire rack for 20 minutes. Sprinkle with the confectioners' sugar, if desired.
Make sure to toss the apples and raisins until they're evenly coated with the flour mixture. The flour helps to thicken the juices released by the apples as they cook.