In a large bowl, whisk together flour, sugar, and salt. Add the cold butter cubes and use a pastry cutter (or your fingertips) to cut the butter into the flour until the mixture resembles coarse crumbs with pea-sized pieces of butter throughout. Do not overwork — visible butter chunks are what create flaky layers.

Add ice water 2 tablespoons at a time, stirring with a fork after each addition, until the dough just barely comes together when you pinch a piece. You may need anywhere from 8–12 tablespoons total. The dough should be shaggy, not smooth.

Turn the dough out onto a lightly floured surface and divide it into two pieces — one slightly larger than the other (about 55% and 45%). Flatten each into a rectangle, wrap tightly in plastic wrap, and refrigerate for at least 1 hour or up to 2 days.

Peel, core, and slice all apples to about ¼-inch thickness. Place in a very large bowl.

Add the brown sugar, granulated sugar, cornstarch, cinnamon, nutmeg, allspice, salt, lemon juice, and lemon zest. Toss thoroughly until every apple slice is evenly coated. Let the mixture sit for 10 minutes — it will release juices and the sugars will dissolve into a glossy coating. This is good.

Taste the filling and adjust spices if desired. Set aside while you roll out the crust.

Preheat your oven to 400°F (205°C). Line a 10×15-inch rimmed jelly roll pan with parchment paper and lightly spray with nonstick cooking spray.

On a lightly floured surface, roll the larger piece of dough into a rectangle roughly 13×18 inches — large enough to cover the bottom and sides of the pan with about 1 inch of overhang. Roll the dough gently around your rolling pin and carefully unroll it over the pan. Press it into the corners and up the sides. Trim any excess, leaving about a ¾-inch overhang around the rim.

Pour the apple filling evenly over the bottom crust, spreading it into an even layer. Dot the top of the filling with the small cubes of butter.

Roll the second piece of dough into an 11×16-inch rectangle and lay it over the top of the filling. Tuck the top crust edges under the overhanging bottom crust and crimp together all around the edge, pressing firmly to seal. Cut about 15–20 steam vents across the top crust using a sharp knife or the tip of scissors.

Whisk together the egg and milk to make the egg wash. Brush the entire top crust generously with the egg wash, making sure to coat the crimped edges too. Sprinkle turbinado sugar evenly over the top.

Bake at 400°F for 20 minutes, then reduce the oven temperature to 375°F and continue baking for 28–32 minutes more, until the crust is deep golden brown and the filling is bubbling actively through the steam vents.

Remove from the oven and let the slab pie cool on a wire rack for at least 30–45 minutes before cutting into squares. This rest time allows the filling to set so it doesn't run when sliced. Cut into 4×5 rows for 20 squares. Serve warm or at room temperature.