• Grease lightly a 9-inch deep dish pie plate and set it aside.

Make the crumble topping.

• In a medium-sized bowl, place the flour, xanthan gum, brown sugar, granulated sugar, cinnamon, nutmeg and salt, and whisk to combine, working out any lumps in the brown sugar.
• Add the butter, and mix to combine well.
• Place the bowl in the refrigerator to chill for about 10 minutes or until firm (or the freezer for about 5 minutes).

Make & shape the crust

• Prepare the pie crust according to the recipe instructions through step 5, chilling the unshaped dough for 30 minutes.
• Preheat your oven to 375°F. Remove the pie dough from the refrigerator.
• Place the dough 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 crust so that only 1/4-inch of excess is overhanging the plate.
• Tuck the 1/4-inch of excess under itself, and crimp the edge gently all the way around the crust. Pierce the bottom of the pie crust with the tines of a fork and place the pie plate in the freezer to chill for 10 minutes.

Par-bake the pie shell.

• Remove the pie plate from the freezer and place a large piece of unbleached parchment paper in the center of the crust. Place enough pie weights or dried beans to fill the pie crust, on top of the paper.
• Place the pie plate with the weights in the center of the preheated oven and bake for 10 minutes.
• Remove the pie plate from the oven, and remove and set aside the paper and pie weights. Brush the bottom and sides of the crust with the egg white, and return the pan to the oven.
• Bake for another 3 minutes, and remove from the oven. Reduce the oven temperature to 350°F.

Make the filling

• In a large bowl, place the prepared apple slices and toss with the sugar, cinnamon and salt. Allow to sit while the pie crust is par-baking.
• When the pie crust has finished baking, transfer the sliced apple filling to the pie crust, leaving behind any rendered liquid from the apples.
• Remove the crumble topping from the refrigerator and break up into large chunks with a fork. Scatter the topping all over the top of the pie in an even layer. Press gently on the crumble topping to ensure that it adheres to the filling.

Make-Ahead Option

• At this point, the unbaked, assembled pie in the parbaked pie crust can be covered securely and frozen until ready to use.
• Just bake from frozen according to the rest of the recipe directions. You may need to add a few minutes to the final baking time.

Bake the pie

• Place the pie in the center of the preheated oven and bake for 45 minutes, or until the crumble topping is evenly golden brown and the apple filling is bubbling around the edges.
• If necessary to prevent the edges from burning, cover the crust with foil during the final 15 minutes of baking.
• Remove the pie from the oven and allow to cool completely before slicing and serving.