Heat oven to 425°F. Lightly spray 13x9-inch pan with nonstick cooking spray. In large bowl, combine all strudel ingredients except pastry sheets; mix well.
On waxed paper or foil-lined work surface, quickly layer 6 pastry sheets, spraying with nonstick cooking spray after stacking each. Fold stack in half to form 12x7-inch rectangle. Place in spray-coated pan. Top with half of apple mixture.
Stack 4 pastry sheets, again spraying each sheet. Fold in half to form 12x7-inch rectangle. Place on top of apples in pan. Top with remaining apples.
Stack remaining 8 pastry sheets, spraying each sheet. Fold in half; place on top of apples. Spray top with cooking spray. If desired, sprinkle with 1/2 teaspoon sugar.
Bake at 425°F. for 5 minutes. Reduce heat to 350°F.; bake 25 to 35 minutes or until deep golden brown. Cool slightly.
In small bowl, combine all topping ingredients; blend well. Refrigerate until serving time. Cut strudel into squares; serve warm or at room temperature with topping.