1. To make gluten-free oat flour*  blend 1 ½ cups certified gluten-free oatmeal in a high powered blender until it is finely ground to the consistency of flour, about 1 minute.
2. Add oat flour, tapioca starch, and gluten free flour blend (1/3 cup rice flour, 1/3 cup tapioca starch, 1/3 cup potato starch, 1 teaspoon xanthan gum) to a bowl. Cube the butter into ¼ inch cubes.  Using your fingers blend in the butter with the flour until it is the size of small peas.  The butter will break apart more when it is rolled.
3. In a one cup fluid measuring cup, add the egg and cider vinegar. Beat with a fork until well blended.  Top up the glass measuring cup to the one cup mark with water.  Stir to fully mix.
4. Make a well in the center of the flour mixture. Add ½ of the egg mixture to the well in the flour.  Using a fork, mix the flour into the egg mixture, until all the liquid is used up.  Add more liquid, one tablespoon at a time until the dough comes together to form a soft ball.  Knead the dough briefly, add the remainder of the liquid.  Form a smooth ball.  Cover with plastic wrap or a cloth.  Refrigerate the dough for 60 minutes.  [At this point the dough can be divided into two equal parts, wrapped in plastic wrap and frozen for later use.]
5. Cut the dough in half. Working with one half of the dough at a time, roll out the dough on a piece of parchment paper.  I use this reusable parchment paper.  Rotate the circle of dough ¼ turn after each roll to make a 1/8^th inch thin circle that is at least 10 to 11 inches in circumference.
6. Lift the dough and place in a greased and floured 9 inch pie plate or cast iron pan.
7. Fill the dough lined pan with a filling. Roll out the second part of the dough and cover the pie

For a double crust pie, bake at 375°F for 30 minutes or until the crust is golden brown and the filling bubbles.  For tarts bake for 15 minutes at 375F.