In a small food processor pulse the rice flour, potato starch and xanthan gum to combine. Working quickly, add the cold diced butter and pulse a dozen times or so until the mixture resembles fine crumbs. Add the ice water one tablespoon at a time until the crust begins to clump together and stand tall against the walls of the processor. (I used 3 tablespoons total).
Transfer the gluten-free tart dough to a work surface and form into two equal balls. Wrap each separately in plastic wrap and pat it down into a thin disk. Chill in the refrigerator for 1 hour, or up to three days in advance.
Preheat the oven to 425 degrees F.
While the crust is chilling, prepare the filling: in a medium mixing bowl, toss together the blueberries, strawberries, corn or arrowroot starch, sugar and lemon juice until coated. Set aside.
Let the dough stand at room temperature for a few minutes before handling. Then, unwrap and lay on a work surface dusted with rice flour. (I rolled out my dough directly on parchment paper to make transferring it to the baking sheet easier. You can anchor it with a can if it begins to slide around while you roll.) Place a piece of plastic wrap or parchment paper over the dough and roll out, working from the center, until it’s about 1/4″-1/8″ thickness and a 9 inch diameter. If the dough feels crumbly, let it warm up slightly, then continue working. Repeat with the second ball. Divide the berries between the two galettes, leaving a 1 to 2 inch lip on the sides. Carefully fold the flaps of dough onto itself in large creases. If it breaks, don’t stress: you can patch it up, or use your hands to gently press the outside back together. I found it helpful to use my knife to loosen the dough flaps and help me fold.
Transfer the gluten-free berry galettes to a baking sheet (lined with parchment, if not already using some). Brush the sides of the crust with the egg wash and sprinkle with additional sugar. Bake in the oven for 35 to 40 minutes, until the filling bubbles and the crust is golden brown.
Allow the galette to cool slightly on the pan (don’t worry if the juices leak). Then cut into slices or serve on a platter with scoops of vanilla ice cream (red, white and blue baby!).