01 - Combine bread flour, whole wheat flour, sourdough starter, and water in a large mixing bowl. Mix until no dry flour remains visible. Cover the bowl and let rest for 45 minutes to allow autolyse.
02 - Add sea salt to the dough and knead briefly until fully incorporated. Cover and let rest for 30 minutes before beginning stretch and folds.
03 - Perform 3 to 4 sets of stretch and fold technique every 30 minutes during the first 2 hours of bulk fermentation to develop gluten structure.
04 - While dough ferments, beat softened cream cheese, granulated sugar, and vanilla extract together until smooth and creamy. Set aside until ready to use.
05 - Gently fold fresh blueberries and lemon zest into the dough during the final stretch and fold set, distributing evenly throughout.
06 - Turn dough onto lightly floured surface and pat into a rectangle. Spread cream cheese mixture evenly over surface, then roll jelly roll style and pinch seams tightly to seal.
07 - Shape dough into tight round or oval. Place seam side up in floured proofing basket. Cover and refrigerate for 6 to 10 hours for overnight cold proof.
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 top, and sprinkle with coarse sugar and extra lemon zest if desired. Bake covered for 25 minutes, then uncover and bake 20 minutes until golden brown and crusty.
10 - Remove bread from oven and transfer to wire rack. Cool completely before slicing to prevent filling from oozing.