Mix the flours and water until no dry flour remains. Cover and rest 40 minutes. This is a standard naturally-leavened schedule. Times depend on your room temperature and how lively your starter is, so judge by the dough rather than the clock.
Add the starter and salt. Squeeze and fold the dough until both are fully worked in.
Bulk ferment 4 to 5 hours at room temperature, folding the dough over itself every 45 minutes for the first 3 hours. It is ready when it has risen by about half and feels airy.
Turn out onto a lightly floured surface. Pre-shape into a round and rest 20 minutes uncovered.
Shape into a tight round or oval and place seam-side up in a floured banneton.
Cover and refrigerate 12 to 16 hours. The cold retard is what develops the flavour and makes the dough easy to score.
Preheat a Dutch oven to 250C (480F) for 45 minutes. Turn the dough out, score it, and bake covered for 20 minutes.
Uncover, drop to 230C (450F), and bake another 20 to 25 minutes until deep brown. Cool at least an hour before slicing; cutting it warm makes the crumb gummy.
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