In a food processor, pulse the rice flour, sorghum flour, tapioca flour, sugar, xanthan gum, and salt to mix well. Add the butter and pulse until pea-sized lumps form. Add the egg and pulse until blended. If necessary, add the cold water, 1 tablespoon at a time, pulsing just until the dough forms a ball, scraping down the sides of the bowl as needed. Scrape the dough onto a sheet of waxed paper. Shape the dough into a ball and then flatten into a disk that’s about ½ inch (12 mm) thick. Cover tightly with the waxed paper and refrigerate until firm, at least 1 hour and up to 1 day.
Dust the dough on both sides with sorghum flour and place between 2 large sheets of waxed paper on a work surface. Roll out the dough through the paper into a 12-inch (30-cm) round, removing the top sheet of paper and turning the dough over occasionally. Remove the top sheet of paper and, using the second sheet of paper, invert the dough into a 9-inch (23-cm) glass pie dish. Gently remove the paper. Using your fingers, press the dough into the dish bottom and sides. Fold over the excess dough on the edges. Using fingertips, crimp the crust all around the edge.