Begin by making the pie crust. In a large bowl, whisk the flour, sugar and salt together until combined. Use a pastry blender, a fork or your fingertips to blend the cold butter cubes into the flour until the pieces are no larger than the size of a pea. Add the water and stir until the dough begins to form, then use your hands to press the dough into a cohesive ball. Divide dough in half, wrap each half in plastic wrap and refrigerate for about 45 minutes to an hour.
Place a rack in the center of the oven and preheat to 375 degrees. Line the bottom of a jelly roll pan (approximately 10" x 15") with parchment paper or spray well with cooking spray and set aside.
In a large bowl, mix the cubed apples, lemon juice, sugar, cornstarch, cinnamon, nutmeg and salt and set aside while you roll out the crust. NOTE: When working with fresh fruit, the amount of thickener you need depends on how ripe the fruit is. If you’re using super sweet, ripe fruit, increase the cornstarch by 1 or 2 Tbsp.
On a lightly floured surface, roll one of the pie crusts (if one half is slightly larger than the other; roll out the larger one first). Roll the dough into a rectangle approximately 18" x 13" in size. Roll the dough around the rolling pin and transfer it to the prepared jellyroll pan. Line the entire bottom and sides of the pan and trim off any overhang.
Pour apple mixture over the crust and spread evenly. Roll the top crust into a 16" x 11" rectangle large enough to cover the top of the pie. Tuck the edges of the top crust between the edge of the bottom crust and the pan, pinching the edges to seal the filling inside. Brush the entire top crust with beaten egg and sprinkle with a light coating of granulated sugar.
Bake until crust is golden brown on top and filling is bubbly, about 40 to 45 minutes. Remove the pie from the oven and cool on a rack until room temperature (or slightly warmer). While the pie is cooling, stir all ingredients for the glaze together in a small bowl. When the pie is barely warm, drizzle the glaze over the crust. Cut into squares and serve warm or at room temperature.
When making the glaze, you can adjust the drizzling consistency to your liking by adding a few drops of milk or a bit more powdered sugar.