01 - Combine bread flour, whole wheat flour, and water in a large bowl. Mix until just incorporated. Cover and let rest for 1 hour to hydrate the flour.
02 - Add sourdough starter and sea salt to the autolysed flour mixture. Mix by hand or spatula until fully incorporated and smooth.
03 - Cover bowl and let dough ferment at room temperature for 4-5 hours. Every 30-45 minutes, perform stretch and folds: grab one edge of dough, pull upward, and fold over center. Rotate bowl and repeat 4 times total.
04 - Turn dough onto lightly floured surface. Gently shape into a round boule. Cover and let rest for 20 minutes to relax gluten.
05 - Shape dough into a tight round loaf, creating surface tension. Place seam side up in a well-floured proofing basket or lined bowl.
06 - Cover and let rise at room temperature for 2-3 hours until increased by 50%. For deeper sour flavor, refrigerate overnight for 8-12 hours.
07 - Place Dutch oven or heavy pot with lid in oven. Preheat to 480°F for at least 30 minutes to ensure proper heat retention.
08 - Gently invert dough onto parchment paper. Score surface with sharp blade in desired pattern. Transfer to preheated pot, cover, and bake for 20 minutes. Remove lid, reduce temperature to 430°F, and bake 25 minutes until deep golden brown with hollow sound when tapped.
09 - Remove bread from pot and transfer to wire rack. Cool completely for at least 2 hours before slicing to prevent gummy texture.