1. Unfold pastry on a lightly floured board. With a floured rolling pin, roll pastry into a neat 10- by 14-inch rectangle.
2. In a bowl, with a mixer, beat cream cheese, egg, powdered sugar, lemon peel, and lemon juice until well blended.
3. Spoon cream cheese mixture down center of longest dimension of pastry to within 1 inch of each end; spread evenly to make a 4-inch-wide band. Fold 1 long pastry edge over filling; brush top of edge with water. Fold remaining edge over filling, overlapping opposite side of pastry. Pinch pastry ends to seal. With 2 wide spatulas, gently lift the roll and lay, seam down, on a lightly buttered 14- by 17-inch baking sheet. With a sharp knife, make shallow slashes at 1-inch intervals across the pastry. Sprinkle pastry evenly with granulated sugar.
4. Bake in a 400° regular or convection oven until strudel is deep golden brown, 25 to 30 minutes. Let cool on pan at least 5 minutes. Serve warm or cool. Cut into 6 to 8 equal slices and transfer to plates.