Make the Crust: Whisk together the flour, sugar and salt in a large bowl. Using a pastry blender, two forks, or your fingertips, work the butter into the flour until the biggest pieces of butter are the size of tiny peas. Gently stir in the water with a rubber spatula, mixing it until a craggy mass forms. Using your hands, knead it two or three times to form a ball. Divide the dough roughly in half (it’s okay if one is slightly larger). Wrap each half in plastic wrap and flatten a bit, like a disc. Chill in the refrigerator for at least an hour (or up to two days).
Preheat oven oven to 375 degrees F. Line the bottom of 10x15x1-inch baking sheet or jellyroll pan with parchment paper.
Prepare the Filling: In a large bowl, toss the apples with the lemon juice until coated. Add the sugar, cornstarch, cinnamon, nutmeg, allspice and salt, and stir to evenly coat.
Assemble the Pie: On a lightly floured surface, roll one of your dough halves (the larger one, if you have two different sizes) into an 18x13-inch rectangle. Transfer the dough to the prepared baking sheet and gently work the dough into the corners and along the edges of the pan. Trim any overhang off the sides to ¾-inch.
Turn the apple mixture onto the dough and spread into an even layer.
Roll the second piece of dough (the smaller one, if they were different sizes) into a 16x11-inch rectangle. Drape over the filling and fold the bottom crust’s overhang over the edges, sealing the two pieces of dough together. Crimp the edges, if desired. Using a paring knife, cut nine small slits over the surface of the top crust.
In a small bowl, whisk together the egg, water and salt. Brush a thin coating of the egg wash evenly over all of the exposed crust.
Bake the Pie: Bake until the crust is golden brown and the filling is bubbling, 40 to 45 minutes. Transfer the pan to a wire rack and cool to room temperature before serving, about 45 minutes. Leftovers will keep at room temperature, well-covered, for up to three days.