01 - Combine bread flour, whole wheat flour, and water in a large mixing bowl. Stir until just incorporated with no dry patches. Cover the bowl and let rest at room temperature for 1 hour to hydrate the flour.
02 - Add the active sourdough starter and sea salt to the hydrated dough. Mix by hand using a pinching motion or use a spatula until fully incorporated, about 3 to 5 minutes.
03 - Cover the bowl and let ferment at room temperature for 4 to 5 hours. Every 30 to 45 minutes, perform a stretch and fold: grab one edge of the dough, pull 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 work surface. Gently shape 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 towel.
06 - Cover and let rise at room temperature for 2 to 3 hours until increased in volume by about 50%. Alternatively, refrigerate overnight for 8 to 12 hours for a more pronounced sour flavor.
07 - Position a Dutch oven or heavy cast-iron pot with its lid on the center rack. Preheat oven to 480°F for at least 30 minutes to thoroughly heat the vessel.
08 - Gently invert the dough onto a piece of parchment paper. Use a sharp blade or bread lame to score the surface with a shallow slash about 1/4 inch deep. Transfer dough to the preheated pot, cover with the lid, and bake for 20 minutes. Remove the lid, reduce temperature to 430°F, and continue baking for 25 minutes until deep golden brown and hollow sounding when tapped.
09 - Remove the bread from the pot and transfer to a wire cooling rack. Let cool completely for at least 2 hours before slicing to allow the crumb structure to set properly.