For the crust
3 3/4 cups (525 g) all purpose gluten free flour (I highly recommend my Better Than Cup4Cup blend), plus more for sprinkling
1 3/4 teaspoons xanthan gum (omit if your blend already contains it)
3/4 teaspoon baking powder
1 teaspoon kosher salt
15 tablespoons (210 g) unsalted butter, chilled and grated
1 1/4 cups (300 g) sour cream, chilled
Ice water by the teaspoonful, as necessary
Egg wash (1 egg + 1 tablespoon water or any kind of milk, beaten), for brushing
For the apple pie filling
12 medium-sized firm apples (about 4 pounds) (a mixture of Granny Smith, yellow, and Macintosh or Empire apples works great), peeled, cored and chopped
1/2 cup (72 g) cornstarch
3/4 cup (164 g) packed light brown sugar
1 teaspoon kosher salt
1 teaspoon ground cinnamon
1 cup (8 fluid ounces) lukewarm water
For the glaze
1/2 cup (58 g) confectioners’ sugar
1 teaspoon lukewarm water, plus more as necessary
First, make the pie crust dough. In a large bowl, place the flour, xanthan gum, baking powder, and salt, and whisk to combine well. Add the grated, chilled butter, and toss to coat it in the dry ingredients. Add the sour cream, and mix to moisten the dry ingredients with the sour cream. The dough should be shaggy and somewhat crumbly. Knead the dough together with clean hands until it begins to come together. Add ice water by the teaspoon only if necessary for the dough to hold together. Divide the dough into two equal portions, turn each piece out onto a separate sheet of plastic wrap, and press into a disk as you close the plastic wrap around the dough. It will still seem rough. Place the two pieces of dough in the refrigerator to chill for 30 minutes.
Once the dough has chilled, turn out one piece of it onto a lightly floured surface. Sprinkle the dough lightly with more flour, and roll it out with a rolling pin into a rectangle that is about 1 inch thick, moving the dough frequently and sprinkling it lightly with flour if it begins to stick. Fold the dough over on itself like you would a business letter. Sprinkle the dough again lightly with flour, and roll out it out once again into a rectangle about 1 inch thick. Once or twice more, sprinkle the dough lightly with flour, roll it out, and fold it over on itself like you would a business letter, then wrap it tightly and place it back in the refrigerator to chill for about 2 hours. Repeat with the other piece of dough. This entire dough preparation can be done up to a week in advance, and stored in the refrigerator provided it is wrapped very tightly.
When the dough is nearly chilled, make the filling. In a medium, heavy-bottom saucepan, place all of the filling ingredients and cook over medium heat, stirring occasionally, until the apples are just fork tender (7 to 10 minutes). Remove from the heat and allow to cool to warm room temperature.
Prepare a 10- x 15- inch jelly roll pan. If your pan is nonstick, don’t oil it. If it isn’t spray it lightly with cooking oil spray and set it aside. As the filling cools, remove one of the pieces of pie crust from the refrigerator. On a very lightly floured surface, roll out the dough into an 18 inch x 12-inch rectangle. It should be about 1/4-inch thick. Trim any especially jagged edges. Fold the crust in half, and lift it carefully onto the prepared jelly roll pan in the proper orientation and into the edges of the pan. Fold any slightly overhung edges under themselves to secure them. Brush the bottom of the crust lightly with the egg wash, and place it in the refrigerator to stay cold. Preheat your oven to 375° F.
Remove the remaining disk of pie crust from the refrigerator, and turn it out onto a lightly floured flat surface. Roll the dough into a 12-inch x 15-inch rectangle. It should be about 1/4-inch thick. Trim any especially jagged edges. Pour the prepared filling on top of the crust, and spread out into an even layer. Lift the top crust onto the top of the pan, fold the top crust over the bottom, and crimp the edges together to seal them, using the knuckle of one hand and the fingertips of the other. Using a sharp knife or pastry wheel, cut small vents all over on the top crust, slicing all the way through to the filling. Brush the top crust liberally with the egg wash.
Place the pan in the center of the preheated oven and bake until the crust is golden brown, about 45 minutes. Transfer to a wire rack to cool completely. When the pie is nearly cool, make the glaze by adding the water slowly to the confectioners’ sugar in a small bowl, stirring constantly until you have a very thick paste. Add more water by the droplet until the glaze is thickly pourable. Drizzle on top of the cooled pie and allow to set briefly before slicing and serving.