Whisk together the flour or flour blend, sugar, 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 eggs and vinegar or lemon juice together till very foamy. Mix into the dry ingredients. Stir until the mixture holds together, adding additional cold water if necessary.
Divide in half, shape each half into a flattened rectangle, wrap well, and chill for an hour, or up to overnight.
Allow the dough to rest at room temperature for 10 to 15 minutes before rolling.
To make the filling: If you're using fresh berries, rinse and drain them. Place the fresh (or frozen) blueberries in a saucepan. Whisk together the sugar, flour or ClearJel, and salt, and add to the berries along with the lemon juice, stirring to combine. Set the pan over medium heat. Cook until the mixture starts to thicken, about 5 minutes. Transfer the cooked berries to a bowl and let cool to room temperature.
Preheat the oven to 425°F; place a rack on the middle shelf. Line a baking sheet with parchment paper.
To assemble the pies: Roll each piece of dough into a 7" x 17 1/2" rectangle. With a straight edge and pastry wheel, cut out ten 3 1/2" squares from each piece.
Divide the filling among 10 of the squares, using about a heaping tablespoon for each. Brush some of the beaten egg along the edges of each filled square.
Cut a vent into the each of the remaining 10 squares, using a decorative cutter, if desired.
Top each filled square with a vented square, and press along the edges with the tines of a fork or a pie crust crimper to seal.
Brush the top of each pie with some of the remaining beaten egg, and sprinkle with sparkling sugar. Transfer the pies to the prepared baking sheet.
Bake the pies for 18 to 20 minutes. Remove them from the oven, and let cool for 20 minutes before serving.
TIPS FROM OUR BAKERS
*Make your own blend Many of our gluten-free recipes use our King Arthur Gluten-Free Multi-Purpose Flour, which includes ingredients that reduce the grittiness sometimes found in gluten-free baked goods. Our flour also increases the shelf life of your treats, keeping them fresh longer.
The following make-at-home blend, featuring stabilized brown rice flour, works pretty well when substituted; and it tastes better than a blend using regular brown rice flour.
Whisk together 6 cups (28 1/2 ounces) King Arthur stabilized brown rice flour; 2 cups (10 3/4 ounces) potato starch; and 1 cup (4 ounces) tapioca flour or tapioca starch. Store airtight at room temperature. Note: You can substitute white rice flour for the brown rice flour if you like; it'll make your baked goods grittier (unless you manage to find a finely ground version).
Why no listed weight for the blueberries? Berries can vary quite a bit in weight, and we found that measuring by volume is best in this case.