01 - Combine bread flour, whole wheat flour, sourdough starter, and water in a large mixing bowl. Stir until no dry flour remains visible. Cover the bowl and let rest for 45 minutes to allow autolyse.
02 - Sprinkle sea salt over the dough surface. Knead briefly by hand until salt is fully distributed. Cover and let rest for 30 minutes.
03 - Perform 3 to 4 sets of stretch-and-fold maneuvers at 30-minute intervals during the first 2 hours of bulk fermentation. Grab one edge of dough, stretch upward, and fold toward center. Rotate bowl and repeat for all sides.
04 - While dough ferments, beat softened cream cheese, granulated sugar, and vanilla extract in a small bowl until completely smooth and creamy. Set aside until ready to use.
05 - During the final stretch-and-fold set, gently fold fresh blueberries and lemon zest into the dough. Use delicate motions to distribute berries without crushing them.
06 - Turn dough onto lightly floured surface. Pat into rectangle approximately 10 by 14 inches. Spread cream cheese filling evenly across surface, leaving 1-inch border. Roll tightly from long end like a jelly roll. Pinch seam firmly to seal.
07 - Shape into round or oval loaf. Place seam-side up in floured proofing basket or bowl lined with floured towel. Cover and refrigerate for 6 to 10 hours for overnight cold proof.
08 - Position oven rack in lower third position. Place Dutch oven or cast iron pot with lid inside. Preheat to 450°F for at least 30 minutes to ensure thorough heating.
09 - Turn dough out onto parchment paper. Score top with razor or sharp knife in desired pattern. Sprinkle with coarse sugar and extra lemon zest if using. Transfer dough and parchment into preheated Dutch oven. Cover with lid and bake for 25 minutes.
10 - Remove Dutch oven lid and continue baking for 20 minutes until deep golden brown and crust sounds hollow when tapped. Internal temperature should reach 200°F.
11 - Transfer loaf to wire rack. Cool for at least 2 hours before slicing to allow internal structure to set. This prevents gummy texture.