01 - Combine bread flour, whole wheat flour, and water in a large bowl. Mix until just incorporated and no dry flour remains. Cover the bowl and let rest for 1 hour at room temperature.
02 - Add the active sourdough starter and sea salt to the autolysed dough. Mix by hand using a pinching motion or use a spatula until fully incorporated and the dough begins to develop some structure.
03 - Cover the bowl and let ferment at room temperature for 4-5 hours. Every 30-45 minutes, perform a stretch and fold: grab one edge of the dough, stretch it upward, and fold it over the center. Rotate the bowl and repeat 4 times per session.
04 - Turn the dough onto a lightly floured surface. Gently shape it into a round boule by folding the edges toward the center. Cover with a towel and let rest for 20 minutes to relax the gluten.
05 - Shape the dough into a tight round loaf, creating surface tension. Place seam side up in a well-floured proofing basket or bowl lined with a floured kitchen towel.
06 - Cover and let rise at room temperature for 2-3 hours until increased in volume by about 50%, or refrigerate overnight for 8-12 hours for a more pronounced sour flavor and improved oven spring.
07 - Place a Dutch oven or heavy cast-iron pot with its lid in the oven. Preheat to 480°F for at least 30 minutes to ensure the pot is thoroughly hot.
08 - Gently invert the dough onto parchment paper. Score the surface with a sharp blade or lame in a pattern of your choice. Transfer to the preheated pot, cover with the lid, and bake for 20 minutes. Remove lid, reduce temperature to 430°F, and bake another 25 minutes until deep golden brown.
09 - Remove the bread from the Dutch oven and transfer to a wire rack. Cool completely for at least 2 hours before slicing to allow the crumb to set properly.