Mix flour, sugar, and salt in a bowl.
Cut in butter until mixture resembles coarse crumbs.
Stir in sour cream until dough forms. Chill for 1 hour.
Roll dough into a rectangle and spread apple pie filling down the center.
Fold sides over filling, sealing edges.
Mix egg with water and brush over pastry.
Bake at 375°F for 30-35 minutes until golden.
Cool slightly and dust with powdered sugar before serving.