In a large bowl, whisk together the flour, sugar, and salt for the crust. Add the cold cubed butter and cut it into the dry ingredients using a pastry blender or your fingers until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

Drizzle 4 tablespoons of ice water over the mixture and gently mix with a spatula. Add more ice water, 1 tablespoon at a time, just until the dough comes together when squeezed. Do not overmix.

Divide the dough in half, shape each half into a flat rectangle, wrap tightly in plastic wrap, and refrigerate for at least 1 hour.

While the dough chills, prepare the filling. In a medium saucepan, melt 1 tablespoon of butter over medium heat. Add the diced apples, brown sugar, granulated sugar, cinnamon, nutmeg, and lemon juice. Cook for 5-7 minutes until apples begin to soften.

Stir in the cornstarch slurry and cook for 1-2 more minutes until the mixture thickens significantly. Remove from heat and let cool completely.

Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. Whisk the egg and milk together in a small bowl for the egg wash.

On a lightly floured surface, roll out one disk of dough into a 10x14-inch rectangle, about 1/8-inch thick. Cut into 8 smaller rectangles (about 3.5x5 inches each). Repeat with the second disk to get a total of 16 rectangles.

Place 8 rectangles on the prepared baking sheet. Spoon about 2 tablespoons of the cooled apple filling onto the center of each. Brush the edges lightly with the egg wash.

Place the remaining 8 rectangles on top. Press the edges firmly to seal, then crimp the edges with a fork. Cut 2-3 small slits in the top of each pie to allow steam to escape.

Brush the tops of the hand pies with the remaining egg wash and sprinkle generously with turbinado sugar.

Bake for 20-25 minutes, or until the hand pies are golden brown and the filling is bubbly. Let cool on the baking sheet for 5 minutes before transferring to a wire rack.