Preheat the oven to 425°F. Leave the pie crusts out to come to room temperature per the directions on the box. Place two of the pie crusts on top of each other on a lightly floured work surface. Using a rolling pin, roll it out to a rough rectangle that is 18x12 inches.
Using a 10x15x1 jelly roll pan, place the crust in the pan, forming it in the pan with the excess crust overlapping the sides. Set aside.
In a large bowl, add the sliced apples, sugar, cornstarch, lemon juice, cinnamon, nutmeg, and salt. Stir together until fully combined. Spoon the apples evenly onto the crust.
Again, roll out the last two pie crusts into an 18x12-inch rectangle. Place the on top of the pan. Tuck the top crust under the bottom and crimp the edges. Slice slits in the top crust so air can escape while baking.
Bake for 30 to 35 minutes, tenting with foil after the first 15 minutes. Bake until golden brown. If you think it needs more browning after 35 minutes, take the foil off and watch closely until your desired color is reached.
Place on a wire rack to cool for 1 hour.
While the pie is cooling, place the milk in a small saucepan and place it on the stove over medium-low heat. Stir occasionally, at the very first sign of any bubbling, take it off the what, and whisk in the butter. Let it cool to room temperature and then whisk in the powdered sugar.
After the pie has cooled for 1 hour, brush it all over with the glaze with a pastry brush. Wait 30 minutes, brush another layer of glaze on. Let cool 30 more minutes, slice and serve.