How to Make It
In a food processor, combine 1 3/4 cups of the all-purpose flour with the rye flour and 3/4 teaspoon of salt. Add the butter and pulse until it is the size of raisins. Add the ice water and pulse until the dough is evenly moistened. Divide the dough into 2 pieces and press into disks. Wrap the disks in plastic and refrigerate for 1 hour.
Preheat the oven to 375°. In a bowl, toss the apples with the granulated sugar, cinnamon, nutmeg, 1/8 teaspoon of salt and the remaining 1 tablespoon of flour.
On a lightly floured surface, roll out one disk of dough into an 11 1/2-inch square, about 1/4 inch thick. Trim off a 1/4-inch border to even the edges. Cut the dough into four 5 1/2-inch squares. Spoon half of the apple filling onto the squares. Fold over the corners to form triangles. Using a fork, crimp the edges to seal. Transfer the turnovers to a parchment paper-lined baking sheet. Repeat with the remaining dough and filling.
Brush the turnovers with the egg white wash and sprinkle generously with the celery-seed sugar. Make two small slits in the top of each turnover. Bake the turnovers for about 35 minutes, until golden all over. Let cool for 15 minutes. Serve warm.