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 the mixture rest for 45 minutes to allow flour hydration.
02 - Add sea salt to the dough and knead briefly until fully incorporated. Cover and let rest for 30 minutes.
03 - Complete 3 to 4 sets of stretch-and-fold technique at 30-minute intervals 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.
05 - Gently fold 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 rectangle. Spread cream cheese mixture evenly over surface. Roll dough tightly jelly-roll style and pinch seams firmly 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 - Place Dutch oven in oven and preheat to 450°F for at least 30 minutes before baking.
09 - Turn dough onto parchment paper. Score top with sharp knife or lame. Sprinkle with coarse sugar and extra lemon zest if using. Transfer to preheated Dutch oven, cover, and bake for 25 minutes.
10 - Remove Dutch oven lid and continue baking for 20 minutes until loaf is deeply golden and crust sounds hollow when tapped.
11 - Transfer bread to wire rack and let cool completely before slicing to preserve texture.