Combine the ice water and buttermilk in a measuring pitcher and chill until needed. In a large bowl, combine the sweet rice, oat, and millet flours with the cornstarch, tapioca flour, ground chia seed, sugar, and salt. Scatter the butter pieces over the top, and work in with a pastry blender or your fingertips until the mixture resembles gravel, with lots of butter chunks the size of peas and almonds.
Drizzle the buttermilk mixture into the flour mixture 1 tablespoon at a time, tossing the dough with a flexible silicone spatula to moisten evenly. Add just enough water for the dough to hold together when you give it a squeeze, and add it directly to the dry floury bits that like to hang out on the bottom of the bowl; you may not need all of the mixture, or you may need to add more ice water.
At this point you can do one of two things:
-Knead the dough in the bowl 10-20 times to bring it together.
-Dump the dough out onto the counter and fraisage by dragging portions of the dough across the counter with the heel of your hand (this makes for a flakier dough).
Either way, gather the dough up into a ball (a metal bench scraper helps if using the fraisage method) wrap it loosely in plastic wrap, and flatten it into a disk. Chill the dough 30 minutes.
Optionally, for extra flake, roll the dough out on a lightly floured surface, dusting on top and underneath the dough as you work to prevent sticking, until it is a scant 1/4" thick; the dimensions don't matter. Fold the dough into thirds like you're folding a letter, then fold again into thirds. The dough may crack, and this is ok. Wrap the dough and chill until cold, at least 30 minutes and up to 1 day.
On a lightly floured surface, press and roll the dough into a square, then cut the square in thirds both ways to make 9 squares. Roll each square into a circle 6" in diameter and 1/8" thick. Use a pizza wheel to trim the dough into an even round, and stack the rounds on a plate, dusted with a bit of flour to prevent sticking. Chill until cold, 30 minutes, or wrap and chill up to 1 day.
Position racks in the center and upper third of the oven and preheat to 425ºF. Line two large, rimmed baking sheets with parchment paper.
Make the filling:
Place the sugar in a medium bowl. Add the vanilla seeds and lemon zest and rub with your fingertips until the sugar is moistened. Whisk in the cornstarch and salt. In a large bowl, combine the prepared rhubarb and strawberries and toss with the rosewater and lemon juice.
Lay the dough rounds on the prepared baking sheets and let stand for a few minutes to soften a bit. Have the egg wash and coarse sugar standing by. Working quickly so the fruit doesn't lose too much juice, toss the fruit mixture with the sugar mixture to coat. Divide the mixture among the dough rounds, mounding it in the center and leaving 1" on all sides; you may not need all the fruit at this point. Fold up the edge of each dough round around the fruit, leaving a 2" window in the center and pleating the dough as you go, and press down to flatten slightly. Tuck in some of the extra fruit, and spoon over any juices left in the bowl. Brush the edges of the dough with the egg wash and sprinkle with the sugar.
Bake the galettes until the dough is golden and crisp and the fruit is bubbling, rotating the pans top to bottom and front to back halfway through, 30-35 minutes total. While still hot, remove the galettes from the baking sheet to a cooling rack. Let cool to warm, then serve with whipped cream or ice cream, if you like.
The galettes are best the day of baking but extras will keep at room temperature for up to 1 day or refrigerated airtight for up to 3 days. Reheat before serving for best results.
Adapted from Sweeter Off the Vine: Fruit Desserts for Every Season, by Yossi Arefi.We find this amount of rosewater to be just right, but if you're sensitive to the flavor, reduce the amount to 2 or 3 teaspoons.I use a 1 1/2 times batch of my flaky, gluten-free pie dough to make 9 galettes. See the original post for step-by-step photos of the fraisage and turning process.Feel free to use your favorite pie dough recipe in its place, or use 3/4 of this recipe if gluten isn't an issue. You'll need enough dough for 1 1/2, 9" single crust pies.Nutritional values are based on one of nine servings.