Mix bread flour, whole wheat flour, and 350 g water. Cover and rest 30 minutes for autolyse.

Stir in starter, remaining 50 g water, and salt until smooth.

Perform 4 sets of stretch and folds at 30-minute intervals over 2 hours, covering the bowl between sets.

Cover and refrigerate 12 to 16 hours for a cold bulk ferment.

Turn dough onto a floured surface, preshape into a round, and bench rest 30 minutes.

Shape tightly into a boule, place seam-side up in a proofing basket, and cold proof 1 to 2 hours while preheating.

Preheat oven with Dutch oven inside to 500°F for 45 minutes.

Score the dough deeply at a 45-degree angle and bake covered at 500°F for 30 minutes.

Uncover, reduce heat to 450°F, and bake 20 to 25 minutes until deep amber and hollow-sounding when tapped.

Cool on a wire rack at least 1 hour before slicing.