Prepare the Crust: In bowl of food processor, combine sorghum flour, sweet rice flour, tapioca starch, granulated sugar and salt. Pulse to combine. Add butter. Pulse to combine until no large pieces of butter remain and mixture resembles coarse breadcrumbs, about 8 to 10 one-second pulses. Add egg. Pulse to combine. Add one tablespoon water. Pulse until dough forms. If dough is dry, add remaining tablespoon water.
Prepare the Filling: Adjust oven rack to middle position and preheat oven to 425°F. Lightly grease 10-inch pie plate and set aside. Remove dough from refrigerator. Allow it to sit on the counter for 10 minutes. Meanwhile, in large bowl, rub together the cornstarch and granulated sugar. Add rhubarb and lemon juice. Stir to combine.
Put It All Together: Place dough onto an 16 1/2- by 12 1/2-inch piece of parchment. Generously white rice flour top of dough. Roll dough into an approximate 14-inch circle. Invert 10-inch pie plate onto center of dough. Slide your hand (between counter and parchment paper) under the parchment paper. In one swift motion, flip dough into pan. Dough will overhang pan. Carefully peel parchment away from dough.