01 - Combine bread flour, whole wheat flour, and water in a large bowl. Mix until just incorporated. Cover bowl and let rest for 1 hour.
02 - Add sourdough starter and sea salt to dough. Mix by hand or spatula until fully incorporated and smooth.
03 - Cover and ferment at room temperature for 4 to 5 hours. Perform stretch and fold every 30 to 45 minutes, repeating 4 times total.
04 - Turn dough onto lightly floured surface. Shape gently into round. Cover and let rest for 20 minutes.
05 - Shape dough into tight round loaf. Place seam side up in well-floured proofing basket or bowl.
06 - Cover and let rise at room temperature for 2 to 3 hours. For pronounced sour flavor, refrigerate overnight.
07 - Place Dutch oven or heavy pot with lid in oven. Preheat to 480°F for at least 30 minutes.
08 - Gently invert dough onto parchment paper. Score surface with sharp blade. Transfer to preheated pot, cover, and bake 20 minutes. Remove lid, reduce temperature to 430°F, and bake 25 minutes until deep golden brown.
09 - Remove bread from pot. Cool completely on wire rack before slicing.