Whisk together the potato starch, oat flour, millet flour, and salt in a medium bowl. Cut in butter or lard using a pastry cutter or your hands. Mix in sourdough starter and add additional water, up to 1/4 cup, to achieve a dough that is not crumbly, mixing and kneading dough as you go. It may be more moist than wheat-based pie dough you are used to working with.
Cover dough and place at cool room temperature for 8 or more hours. When ready to use, divide dough in two. Press bottom crust into pie pan using your fingers. Dust a work surface with potato starch or tapioca flour and roll top crust out, adding more flour as needed.