Lightly grease a 9" pie pan., Whisk together the flour or flour blend, sugar, Instant ClearJel, xanthan gum, and salt., Cut the cold butter into pats, then work the pats into the flour mixture until it's crumbly, with some larger, pea-sized chunks of butter remaining., Whisk the egg and vinegar or lemon juice together until very foamy. Mix into the dry ingredients. Stir until the mixture holds together, adding 1 to 3 additional tablespoons cold water if necessary., Shape into a ball and chill for an hour, or up to overnight., Allow the dough to rest at room temperature for 10 to 15 minutes before rolling., Roll out on a piece of plastic wrap, on a silicone rolling mat, or in a pie bag that's been heavily sprinkled with gluten-free flour or flour blend. Invert the crust into the prepared pie pan., Fill and bake as your pie recipe directs.