Mix dough ingredients:

In a large mixing bowl or the bowl of a stand mixer, add the gluten-free flour, salt, unfed sourdough starter, and water at room temperature. Stir until everything is incorporated and no dry bits remain. The mixture will be thick, sticky, and resemble a batter more than standard bread dough. Scrape the bowl sides, lightly smooth the top, and cover.

Ferment until puffy:

Allow the dough to ferment at room temperature in a place without drafts for 6 to 9 hours, adjusting based on your kitchen's warmth. It is ready when it looks puffy, slightly domed, with small bubbles just under the surface, and has expanded by about 50 to 75%. Do not let it double in size, as gluten-free dough may deflate if over-proofed. If the center starts to sink, move to the next step right away.

Shape dough round:

Dust your hands with flour and gently turn the dough onto a floured surface. Form a round by folding the edges inward. The dough should be light and airy yet hold its shape. Place it seam-side up into a well-floured banneton or a bowl lined with a floured kitchen towel. Cover tightly.

Cold proof overnight:

Refrigerate the covered dough for 12 to 16 hours for a cold proof. This slow fermentation controls yeast activity, strengthens the structure, and enhances taste. Do not omit this step.

Preheat and score:

Set a Dutch oven with its lid inside the oven and preheat to 500°F (260°C) for at least 30 minutes. Take the dough from the fridge and turn it out onto a lightly floured parchment sheet, seam-side down. Heavily dust the top with flour, then make a slash about 3/4 to 1 inch deep using a sharp knife or bread lame.

Bake covered then uncovered:

Carefully extract the hot Dutch oven. Using the parchment, lift the dough and lower it into the pot. Cover, return to the oven, and immediately drop the temperature to 450°F (230°C). Bake covered for 40 minutes. Then remove the lid, lower the temperature to 425°F (220°C), and bake for another 30 to 35 minutes until deep golden brown and the loaf sounds hollow when tapped on the bottom. Take the bread out of the Dutch oven and set on a wire rack. Cool fully for at least 2 hours before cutting.