Drop a small spoonful of starter into water — if it floats, it's ready. Also look for: bubbles throughout (not just on top), a domed or slightly fallen top, and a pleasantly sour, yeasty smell. GF starters often don't double the way wheat starters do — look for at least 50% volume increase and visible bubbles.

Whisk together the brown rice flour, white rice flour, sorghum flour, tapioca starch, psyllium husk powder, and salt in a large bowl until fully uniform.

In a separate bowl, whisk the warm water, starter, olive oil, and apple cider vinegar together. Add the wet mixture to the dry, and mix vigorously with a dough whisk or stiff spatula for 2–3 minutes until smooth and uniform. The dough will thicken significantly over 2–3 minutes as the psyllium activates.

Transfer dough to a lightly oiled bowl, cover tightly with plastic wrap, and ferment at room temperature. GF bulk fermentation is a hands-off process — no folds, no punching down. The psyllium gel forms a structure that doesn't benefit from manipulation. Look for 40–50% volume increase and visible surface bubbles.

Generously dust a round banneton with white rice flour (not tapioca starch — it clumps). The dough will stick badly if the basket isn't well floured. You can also line the banneton with a linen cloth dusted with rice flour.

Using a wet bench scraper and wet hands, gently transfer the dough from the bowl into the banneton. Wet hands prevent sticking without adding flour. Smooth the top (which will become the bottom of the baked loaf) as evenly as possible. Don't worry about building tension — the psyllium gel holds the shape.

Cover the banneton with a clean shower cap or plastic wrap and refrigerate for 8–12 hours (overnight is perfect). Cold cold retard slows fermentation to a crawl, allows more complex flavors to develop, and makes the cold dough much easier to score cleanly before baking.

Place the Dutch oven (with lid) on the center rack. Heat oven to 500°F (260°C). Preheat for at least 45 minutes — the vessel needs to be fully saturated with heat for proper crust development.

Remove the banneton from the fridge. Cut a piece of parchment to fit inside the Dutch oven. Invert the dough onto the parchment, then score immediately with a lame or sharp serrated knife at a 45-degree angle, ½ inch deep. A single curved slash or an X pattern works well. Cold dough scores cleanly — this is why we retarded.

Lower the parchment and dough into the preheated Dutch oven using the parchment as a sling. Cover and bake at 500°F (260°C) for 20 minutes. Remove lid, reduce heat to 450°F (230°C), and bake an additional 30–35 minutes until deep golden brown. Internal temperature should read 208–212°F (98–100°C).

Transfer to a wire rack and cool for a minimum of 2 hours — 3 hours is better for GF sourdough. The interior crumb continues to set long after the crust forms. Cutting early produces a gummy, wet interior even in a fully baked loaf.