Feed your sourdough starter 4–6 hours ahead; it should double and pass the float test.

In a large bowl, mix 500 g bread flour, 100 g whole wheat flour, and 375 g water until combined. Let rest for 30 minutes (autolyse).

Add 100 g starter and 10 g salt. Pinch and fold the dough until incorporated.

Bulk ferment at room temp for 4–6 hours. Stretch and fold every 30 minutes for the first 2 hours.

Shape the dough into a boule or batard. Place in a floured banneton. Refrigerate overnight.

Preheat oven to 475°F with Dutch oven inside. Score the dough, bake covered for 20 minutes, then uncovered for 25–30 minutes.

Cool completely before slicing and serving.