In a clean jar, mix 1/3 cup starter, 1/3 cup flour, and 3 Tbsp water. Cover and let sit at ~75 °F until doubled (4–6 hr).

In a large bowl, stir together 3 cups flour and 1⅛ cups water until no dry bits remain. Cover and rest 30 min.

Add 1/3 cup active starter; knead gently for 2 min to combine. Cover and rest 30 min.

Sprinkle salt over dough; knead 2 min to incorporate.

Every 30 min, wet your hands and perform one coil fold: lift one side of the dough and fold it into the center; rotate the bowl and repeat on all sides. Cover after each fold.

Repeat for a total of 3 folds (including the one after adding salt). After the last fold, rest 1½ hr at room temperature.

Lightly dust a proofing basket (banneton) with tapioca starch.

Turn dough onto a floured surface; shape into a tight round by folding edges toward the center and dragging to create surface tension.

Place seam-side up in basket, cover, and refrigerate 12–16 hr.

Preheat oven to 500 °F with a Dutch oven inside for 30 min.

Carefully turn proofed dough into the hot pot, seam-side down. Score top with a sharp blade.

Bake covered 20 min, then remove lid, reduce oven to 425 °F, and bake 30 min more until deep golden.

Let cool on a rack at least 90 min before slicing.