Thaw puff pastry according to package directions.

Preheat oven to 375°F. Spray 2 baking sheets with nonstick cooking spray.

Combine sour cream and egg yolk in small bowl; set aside. In separate small bowl, mix egg white and water; set aside.

On lightly floured surface, roll 1 sheet pastry into 12X10-inch rectangle. Spread half of apple pie filling down center 1/3 of pastry. Spread 1/2 cup sour cream mixture over filling; sprinkle with 1/2 cup walnuts.

Fold one long side of pastry over filling mixture; fold other side over filling mixture, overlapping edges. Press edges together to seal. Place on baking sheet, seam side down, tucking ends under. Using sharp knife, make 7 diagonal slits across top; brush with egg white mixture. Repeat with remaining pastry, apple pie filling, sour cream mixture and walnuts. Bake 30 to 35 minutes or until golden brown.