Combine starter, water, flour and salt in a large bowl. Mix with your hand until no dry flour remains, 2 minutes. Cover, rest 30 min.

First fold: wet your hand, grab an edge of the dough, stretch it up and fold over. Rotate bowl a quarter turn. Repeat 3 times. Cover, rest 30 min.

Do 3 more rounds of folds, 30 min apart. The dough should feel stronger each time.

Bulk ferment another 2-3 hours at room temp (total 4-5h from mixing). Dough should be puffy and jiggly but not doubled.

Turn dough onto a lightly floured bench. Fold top down, bottom up, sides in. Flip seam-down. Shape into a tight round by rotating with the sides of your hands.

Place seam-up in a floured banneton or a bowl lined with a floured tea towel. Cover. Refrigerate 12-18 hours.

Put a cast-iron Dutch oven in the oven. Preheat to 240°C fan-forced for at least 45 min.

Turn dough out onto baking paper. Score the top with a blade or sharp knife. Carefully lift paper and dough into the hot pot.

Cover, bake 20 min. Remove lid, reduce to 220°C, bake another 20-25 min until deep brown and hollow-sounding when tapped.

Cool on a wire rack at least 45 min before slicing.