Whisk starter into warm water; add flour and salt. Mix to a shaggy dough; rest 30 minutes covered.
Perform 3 to 4 sets of stretch and folds, 30 minutes apart, over 2 hours.
Bulk ferment covered at room temperature 4 to 6 hours, until grown 50 to 75 percent and bubbly.
Shape into a tight round on a floured surface; rest 15 minutes and tighten again.
Place seam-side up in a floured proofing basket; refrigerate covered 12 to 16 hours.
Preheat oven to 450°F (230°C) with Dutch oven inside for 45 minutes.
Turn dough onto parchment; score 1/4 inch deep.
Bake covered 25 minutes; uncover and bake 15 to 20 minutes more until deep golden brown.
Cool on a wire rack at least 1 hour before slicing.