In a large bowl, mix 500g flour and 350g warm water until just combined. Cover and let rest (autolyse) for 30 minutes.
Add 100g active starter and 10g salt. Mix by hand or with a dough scraper until smooth (~5 minutes).
Cover and let rise at room temperature (70–75°F/21–24°C) for 4–6 hours, folding the dough every 30 minutes for the first 2 hours.
Turn dough onto a floured surface, shape into a round or oval, and place seam-side up in a floured banneton or bowl.
Cover and refrigerate for 12–24 hours (cold proof) OR proof at room temp for 1–2 hours until puffy.
Preheat Dutch oven at 475°F (245°C) for 30 minutes.
Score dough, transfer to the Dutch oven, and bake covered for 25 minutes.
Uncover and bake 20–25 minutes until deep golden. Cool on a wire rack for 1 hour before slicing.
