In a large bowl, place the flour, xanthan gum, baking powder and salt, and whisk to combine well. Add the chopped and chilled butter, and toss to coat it in the dry ingredients.

Flatten each chunk of butter between your thumb and forefinger.

Add 3/4 cup of water and mix until the dough begins to come together.

Add the remaining water by the teaspoon until no part of the dough is crumbly.

Turn out the dough onto a lightly floured piece of unbleached parchment paper and press into a disk.

Sprinkle the dough lightly with more flour, and roll it out 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 the dough once again into a rectangle about 1 inch thick.

Twice more, remove the top piece of parchment paper, sprinkle lightly with flour, and fold the dough over on itself like you would a business letter.

This will smooth out the dough and make it quite easy to handle. Divide the dough into two equal parts, wrap each separately in plastic wrap and place in the refrigerator to chill.

Preheat your oven to 375°F.

In a large bowl, place the apples, sugar, salt, tapioca starch, and ground cinnamon, and toss to coat the apples evenly. Set the filling aside.

Assemble the bottom crust and filling.

Grease a 9-inch metal pie plate generously and set aside. Remove one half of the pie crust from the refrigerator.

Place on a lightly floured piece of unbleached parchment paper, dust lightly with flour, and roll into a 12-inch round, about 3/8-inch thick.

Roll the pie crust loosely on the rolling pin and then unroll it over the prepared pie plate.

Press the pie crust gently into the bottom and up the sides of the pie plate and, with kitchen shears, trim the excess crust so that only 1/4-inch of excess is overhanging the plate.

Tuck the 1/4-inch of excess either under the bottom crust edge and crimp the edge gently all the way around the crust.

Arrange the apples filling on the bottom crust in concentric circles, overlapping one apple slice slightly over the previous one.

Remove the other half of pie crust from the refrigerator.

Place on a lightly floured piece of unbleached parchment paper, dust lightly with flour, and roll it into a rough round about 3/8-inch thick and trim to 10-inches in diameter.

Carefully lift the top crust and center it over the pie plate.

Tuck the very edge of the pie crust under either the bottom crust or under itself.

Crimp the edge all the way around and press together to seal the top and bottom crusts. Place it in the freezer for 10 minutes (or the refrigerator for 30 minutes) to chill until firm.

Brush the top crust and edges of the chilled pie generously with the optional egg wash.

Place the paper bag open on its side, and slide the assembled and chilled pie inside so that the back edge of the pie is about 1-inch away from the bottom of the bag.

Place the pie in the bag on a large rimmed baking sheet and place in the center of the preheated oven. Bake for 1 hour.

Remove from the oven and cut a large hole in the top of the to expose the top and sides of the pie.

Return to the oven and bake until the pie is golden brown all over (about another 10 minutes).

Remove from the oven, and allow to cool slightly so the filling sets before slicing and serving.