Autolyze (optional): Mix flours + water. Rest 30-45 min.
Add starter, rest 30 min.
Add salt, fold in well.
Bulk ferment 4-6 hrs at ~75°F, doing 3-4 stretch + folds early on. Stop when dough is risen 60-70%.
Pre-shape, rest 20 min.
Final shape, flour lightly, place in banneton.
Cold proof 8-12 hrs in fridge.
Bake at 450°F:
20 min covered (Dutch oven)
Cool at least 1 hr before slicing.
