Mix 500g flour and 400g water until no dry spots remain; cover and let rest 30 minutes. Add 100g starter and 10g salt, mix until combined.

Perform 4 sets of stretch and folds at 30-minute intervals (2 hours total), then let rise at room temperature until doubled, 6 to 8 hours.

Turn dough out, preshape into a round, rest 20 minutes, then shape into a boule or batard and place seam-side up in a floured banneton. Cover and proof overnight in the fridge or 1 to 2 hours at room temperature.

Preheat oven with a Dutch oven inside to 450°F (230°C) for 30 minutes. Turn dough onto parchment, score, and bake covered 30 minutes. Uncover and bake 15 to 20 minutes more until deep brown.

Cool loaf on a rack at least 2 hours, then slice with a serrated knife.