Preheat oven to 375°F (190°C).
In a large bowl, mix 2 ½ cups flour and 1 teaspoon salt. Cut in butter until mixture resembles coarse crumbs. Gradually add ice water, tossing with a fork until dough holds together when pressed. Divide dough in half. Shape each into a rectangle; wrap and refrigerate for 1 hour or overnight.
On a lightly floured surface, roll one portion of dough into an 18x13-inch rectangle. Transfer to a 15x10x1-inch baking pan. Press onto the bottom and up the sides of the pan.
In a large bowl, combine apples, granulated sugar, ¼ cup flour, cinnamon, nutmeg, and lemon juice. Spoon over crust.
Roll remaining dough into a 16x11-inch rectangle. Place over filling. Fold bottom crust over edge of top crust; seal and flute edges. Cut slits in top.
Bake 45-50 minutes or until crust is golden brown and filling is bubbly. Cool on a wire rack.
In a small bowl, mix powdered sugar, maple syrup, and enough milk to reach desired consistency. Drizzle over pie.