1. Weigh or lightly spoon flours and cornstarch into dry measuring cups; level with a knife. Sift together flours, cornstarch, granulated sugar, xanthan gum, and salt in a medium bowl.
2. Place cream cheese, softened butter, milk, and egg in a food processor; process until blended. Add flour mixture; process until blended and dough pulls away from sides. Shape dough into a ball; wrap in plastic wrap. Chill 1 hour.
3. Preheat oven to 375°.
4. Combine apple and next 6 ingredients (through cinnamon), tossing to coat apple.
5. Place dough on a baking sheet lined with parchment paper and lightly floured. Cover with an additional sheet of parchment paper. Roll dough into a 13-inch circle; remove top layer of parchment paper. Spoon apple mixture onto half of circle, leaving a 1-inch border around outside edges.
6. Fold dough over filling; press edges together with a fork to seal. Brush top of dough with melted butter; sprinkle with turbinado sugar.
7. Cut three diagonal slits into top of strudel using a sharp knife. Bake at 375° for 40 minutes or until golden brown. Cool completely on pan. Sprinkle with powdered sugar before serving.