In a small food processor, pulse Rice Flour (1 cup), Potato Starch (3 tablespoon), and Xanthan Gum (1/4 teaspoon) to combine. Add the Unsalted Butter (1/2 cup) and pulse a dozen times or so until the mixture resembles fine crumbs.
Add the Water (3 tablespoon) one tablespoon at a time until the crust begins to clump together and stand tall against the walls of the processor.
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 (220 degrees C).
While the crust is chilling, prepare the filling: In a medium mixing bowl, toss together the Fresh Blueberries (1/2 pint), Fresh Strawberry (1 pint), Corn Starch (1 teaspoon), Coconut Sugar (1 tablespoon), and Lemon (1 tablespoon) until coated.
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. Place a piece of plastic wrap or parchment paper over the dough and roll out until its about 1/4 - 1/8" thickness and 9" diameter.
Repeat with the second ball. Divide the berries between the two galettes, leaving a 1-2 inch lip on the sides. Carefully fold the flaps of the dough onto itself in large creases.
Transfer the gluten-free berry galettes to a baking sheet lined with parchment. Beat the Egg (1) with Water (1 tablespoon). Brush the sides of the crust with the egg wash and sprinkle with additional sugar.
Bake in the oven for 35-40 minutes, until the filling bubbles.
Allow the galette to cool slightly on the pan, don't worry if the juice leaks. Then cut into slices or serve on a platter with scoops of vanilla ice cream.