Prepare the pie dough. In a large bowl combine the sweet rice flour, buckwheat flour, oat flour, cornstarch, tapioca starch, ground chia seeds, sugar, and salt.

Scatter the butter over the top and work in with your finger tips until the largest pieces of butter are about the size of a large pea.

Combine the buttermilk and ice water, and drizzle over the flour mixture, 1 tablespoon at a time, tossing the dough with a flexible spatula to evenly moisten. Add just enough water for the dough to hold together when pinched between your fingers.

Knead the dough until it comes together. Dust your surface with buckwheat flour and roll out the dough until it is about ¼-inch thick, lifting and turning the dough, while dusting underneath to keep it from sticking. Fold the dough into thirds, as you would a letter, then fold it in thirds again the other way. Gently press to flatten the dough, wrap it in plastic, and chill for 30-60 minutes.

(Optional, but will create the flakiest crust): Repeat this turning method one more time. Roll the dough until it is ¼-inch thick, lifting and turning the dough, while dusting underneath to keep it from sticking. Fold the dough into thirds, as you would a letter, then fold it in thirds again the other way. Gently press to flatten the dough, rewrap it, and chill for another 30-60 minutes.

Heat the olive oil in a large skillet over low heat and place a cup full of water nearby. Add the onion to the skillet with a pinch of salt. Cook, stirring, until the onion begins to caramelize. Stop stirring and increase the heat to medium-low. Once a brown glaze has formed on the bottom of the skillet, pour 2-3 tablespoons of water into the pan to deglaze it, using a spatula to scrape up any brown bits and stirring them into the onion.Repeat this process until the onion has been thoroughly caramelized, 25-35 minutes. Set aside.

Preheat the oven to 400°F and line a large baking sheet with parchment paper.

Cut the dough into eight equal pieces. Dust your work surface with buckwheat flour and roll each portion into a 6-inch round circle, scant ⅛-inch thick, flipping and dusting with buckwheat flour as you go to keep it from sticking. Trim the edges as needed to make it round and stack them on a plate, dusting them a bit with flour to prevent sticking.

On the prepared baking sheet, distribute the cheese evenly onto each dough round, leaving a 1-inch border around the outside. Divide the caramelized onion among the rounds.

Slice the apple from the core and slice each piece into very thin slices. Fan ¼ of each apple (about 8 slices) on top of the onion and top with salt and pepper.

Fold the excess dough on top of the ingredients, creasing and pleating as you go. If it cracks, just press it back together. Shuffle the galettes to fit evenly on the baking sheet. Brush the dough lightly with cream or milk.

Bake the galettes until the dough is golden, 30-40 minutes, rotating the pan after 20 minutes.

Let galettes cool for 10 minutes, sprinkle with fresh thyme, and serve.