In a large bowl, combine the cassava flour, tapioca flour, xanthan gum, and kosher salt. Stir until incorporated. Add the cold butter or ghee and use your hands to break it apart into the flour mixture until you have an even and crumbly consistency.
Add the whisked egg and knead until you can form the dough into a ball. If your dough is too dry, add 1 or 2 tablespoons of cold water at a time. If your dough is too wet, add 1 or 2 tablespoons of cassava flour at a time. Cover the bowl and refrigerate for at least 30 minutes or overnight.
Remove the dough from the fridge and roll it between two pieces of parchment paper dusted with cassava flour. Aim for an even thickness of around ¼-inch thick. If the dough crumbles, patch the cracks by pressing the dough with your fingers.
Keep the dough between the parchment paper, place it onto a cutting board or similar surface and refrigerate until ready to use.
Preheat the oven to 375F.
Very carefully transfer the dough to a 10-inch pie dish. You can further mend the cracks directly in the pan. Use a fork to make holes in the bottom of the dough. Bake the unfilled dough for 15 minutes. Remove and let the pie dish cool on a wire rack while you prepare the filling.