01 - Combine bread flour, whole wheat flour, sourdough starter, and water in a large bowl. Mix until no dry flour remains. Cover the bowl and let autolyse for 45 minutes to develop gluten structure.
02 - Add fine sea salt to the dough and knead briefly until fully incorporated. Cover and let rest for 30 minutes.
03 - Perform 3 to 4 sets of stretch-and-folds every 30 minutes during the first 2 hours of bulk fermentation. This strengthens the gluten network and develops structure.
04 - While dough ferments, beat softened cream cheese with granulated sugar and vanilla extract until smooth and fully combined. Set aside until ready to use.
05 - During the final stretch-and-fold set, gently fold fresh blueberries and lemon zest into the dough. Handle delicately to avoid crushing the berries.
06 - Turn dough onto a lightly floured surface and pat into a rectangle. Spread cream cheese mixture evenly over the surface. Roll the dough tightly jelly-roll style and pinch seams firmly to seal.
07 - Shape into a tight round or oval. Place seam-side up in a floured proofing basket. Cover and refrigerate for 6 to 10 hours (overnight) for cold proofing.
08 - Preheat oven to 450°F with Dutch oven inside for at least 30 minutes to ensure thorough heating.
09 - Turn dough onto parchment paper. Score the top with a sharp knife or lame. Sprinkle with coarse sugar and additional lemon zest if desired.
10 - Transfer dough with parchment into preheated Dutch oven. Bake covered for 25 minutes to create steam and oven spring.
11 - Remove Dutch oven lid and continue baking for 20 minutes until deeply golden and crusty. Internal temperature should reach 200°F.
12 - Remove bread from oven and transfer to a wire rack. Cool completely before slicing to prevent filling from melting out.