Whisk together the gluten free flour, baking powder and salt in a medium mixing bowl. Add the very cold butter cubes to the flour and use a pastry blender to cut the butter into the flour until all of the flour is coated. They should be larger than peas but not very large clumps.
Combine 1 cup of water with ice and allow it to sit for about a minute to let the water get very cold then measure out 1 cup from there. Add the cold water 2 tablespoons at a time to the butter and flour and mix with a wooden spoon or rubber spatula after each addition. Stop adding water when the dough comes together easily and starts to clump. The dough will be a little sticky but shouldn’t stick to your fingers.
Place the dough on a lightly floured work surface and gently fold the dough onto itself until the flour is fully incorporated into the butter. Form the dough into a ball and cut the dough in half. Form each half into a disc and wrap each in plastic wrap. Refrigerate for at least 2 hours or up to 5 days. This would be a great time to make this recipe in advance by prepping the dough.
When you’re ready to make the pie, preheat the oven to 400°F. Mix together the filling ingredients in a large bowl and stir really well to combine. Set the filling aside to rest while you roll out the dough.
Remove one dough disc from the fridge and roll it out into a 12 inch circle between two pieces of parchment paper. Start in the center of the dough and work your way out with a rolling pin, turning the dough slightly between each roll to create an even circle.
Flip a 9-inch pie dish upside down in the center of one piece of dough then carefully flip both over so the crust is inside the pan. You can also use a rolling pin to roll the dough up and then unroll onto the pie pan.
Spoon the apple filling inside the dough and discard any extra juices from the bottom of the bowl.
Remove the other disc of chilled dough from the fridge and roll into another 12 inch circle between parchment paper. Cut the dough into 12 strips and lattice the pie on top of the filling. See notes for lattice instructions. Once the lattice is done, crimp the edges all around the pie to seal. Lightly brush the top of the pie with the egg wash.
Place the pie onto a large baking sheet and bake for 25 minutes. Keeping the pie in the oven, turn the temperature down to 375°F (190°C) and bake for an additional 30-35 minutes. After the first 20 minutes of bake time, I place a pie crust shield on top of the pie to prevent the edges from browning too quickly.
Allow to cool for at least 2 hours before serving.