Make the dough:

1. In a large bowl, combine the rice, oat, buckwheat flour, cornstarch, tapioca flour, ground chia seed, and salt. Scatter the butter pieces over the top. Use a pastry blender or your fingers to incorporate the butter until the mixture resembles gravel, with lots of butter chunks the size of large peas.
2. Drizzle the apple cider vinegar over the flour mixture, then drizzle the ice water 1 tablespoon at a time, tossing the dough with a rubber 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 need up to 8 tablespoons of water, total.

Use the “fraisage method” to form the dough:

1. Dump the dough onto the counter. Grab a handful of dough, place it on the counter, put the heel of your hand on the dough, and push it away from you, scraping it across the surface several inches. Use a bench scraper to scrape the dough off the counter and place it back in the bowl. Repeat with the remaining dough. It should only take a minute or two to complete this process.
2. Gather all of the dough, then divide into two equal portions. Roll each portion into a ball. Wrap each loosely in plastic wrap or a beeswax wrap, and flatten it into a disc shape. Chill the dough until firm, 30-60 minutes.

Roll the dough:

1. On a lightly floured surface, roll each ball of chilled dough into a rough square that is about 1/4″ thick. Fold it in thirds like you’re folding a letter, then in thirds again, so the dough is a small, layered rectangle. Gently press to flatten it slightly, and chill for another 30 minutes.

Shape the dough:

1. Remove one dough ball from the fridge, unwrap it, and place it on a lightly floured surface. Roll the dough into a circle. You want the diameter of the circle to be about 2 inches bigger than the diameter of your pie plate. Dust the dough lightly with oat flour as needed, rotating and flipping it to prevent it from sticking.
2. Ease the dough into a glass pie plate, fit it into the corners, and trim the overhang. Prick the bottom of the crust all over with the tines of a fork. Chill the crust for 20 minutes, then freeze it for at least 20 minutes, until solid.

Prepare the filling:

1. Combine all the ingredients in a large mixing bowl, and stir until cornstarch dissolves.

To assemble:

1. Transfer the filling to the pie, and keep chilled while you design your top crust. Alternatively, you can roll the dough similar to the base, and cover the pie completely. When adhering top crust to the edges, you can use an egg wash, heavy cream or milk to help the dough adhere.
2. Chill the completed pie while the oven preheats.

To bake:

1. Preheat the oven to 425°F. Bake the pie on a baking sheet for 20 minutes. Reduce the oven temperature to 375°F, and continue to bake for 40-50 minutes more, until the crust is golden and the berry-filling bubbles in the center. If the crust starts to brown too quickly, cover the pie with aluminum foil to prevent excessive browning. Remove from baking sheet to a wire rack; cool 1 hour before serving.
2. Enjoy!