Make the dough. In a large bowl, whisk whole-wheat flour, all-purpose flour, sugar, and salt. Cut in the cold butter with a pastry cutter until pea-sized bits form. Drizzle in ice water and vinegar, tossing with a fork until the dough just comes together. If dry, add a teaspoon more water at a time.

Chill it. Divide dough into two disks, wrap, and refrigerate for 45–60 minutes. This hydrates the flour and firms the butter—aka flake insurance.

Cook the filling. In a skillet over medium heat, combine apples, brown sugar, lemon juice, cinnamon, nutmeg, allspice, and salt. Cook 5–7 minutes until apples soften slightly and release juices. Sprinkle in cornstarch, stir 1–2 minutes until thick and glossy. Remove from heat, stir in vanilla, and cool completely.

Preheat and prep. Heat oven to 400°F (205°C). Line two baking sheets with parchment. Set egg wash and coarse sugar nearby like a little garnish station.

Roll the dough. On a lightly floured surface, roll one disk to about 1/8-inch thick. Trim edges for clean lines, then cut into rectangles (about 4×6 inches) or squares (about 5×5 inches). Repeat with second disk.

Fill the turnovers. Spoon 2–3 tablespoons of cooled apple filling onto half of each piece, leaving a 1/2-inch border. Do not overstuff unless you like sweet lava on your sheet pan.

Seal and crimp. Brush edges lightly with water. Fold dough over to form rectangles or triangles. Press edges to seal, then crimp with a fork. Cut 2–3 small slits on top for steam vents.

Egg wash and sparkle. Brush tops with egg wash and sprinkle with coarse sugar. Transfer to the lined baking sheets, spacing evenly.

Bake. Bake 18–22 minutes until golden brown and crisp, rotating pans halfway. The aroma will convince your neighbors you opened a bakery.

Cool (a little). Let turnovers rest 10 minutes. The filling sets, the crust firms, and your tongue appreciates the patience. Serve warm.