How to Make It
To prepare crust, lightly spoon 1 cup flour into a dry measuring cup; level with a knife. Combine 1 cup flour and 1/8 teaspoon salt in a medium bowl; cut in 3 tablespoons butter with a pastry blender or 2 knives until mixture resembles coarse meal. Add 3 1/2 tablespoons ice water; stir just until moist. Turn dough out onto a heavily floured surface; knead lightly 5 times. Divide dough into 2 equal portions. Place each dough portion between 2 sheets of plastic wrap; roll each dough portion, still covered, into an 8-inch circle. Chill 10 minutes. Uncover dough; place dough circles on a baking sheet lined with parchment paper.
Preheat oven to 350°.
To prepare filling, combine apple and juice; toss to coat. Add 1/4 cup sugar, 1 tablespoon flour, 1/8 teaspoon salt, and 1/8 teaspoon cinnamon; toss to combine. Place half of apple mixture in the center of each dough circle, leaving a 2-inch border; fold edges of dough toward center, pressing gently to seal (dough will only partially cover apple mixture). Combine 1 tablespoon water and egg white; brush mixture gently over outside of crust.
To prepare topping, combine 2 tablespoons flour, 1 tablespoon sugar, 1/8 teaspoon salt, and 1/8 teaspoon cinnamon; cut in 1 1/2 teaspoons butter with a pastry blender or 2 knives until mixture resembles coarse meal. Sprinkle mixture evenly over apple mixture in the center of each croustade. Bake at 350° for 45 minutes or until golden.