01 - Combine bread flour, whole wheat flour, sourdough starter, and water in a large bowl. Stir until no dry flour remains visible. Cover the bowl and let rest for 45 minutes for autolyse.
02 - Add sea salt to the dough and knead briefly until fully integrated. Cover again and allow the dough to rest for 30 minutes.
03 - Perform 3 to 4 sets of stretch-and-fold technique every 30 minutes during the first 2 hours of bulk fermentation. This develops gluten strength.
04 - While dough ferments, beat softened cream cheese, granulated sugar, and vanilla extract together until completely smooth and creamy.
05 - During the final stretch-and-fold set, gently fold blueberries and lemon zest into the dough being careful not to crush the berries.
06 - Turn dough onto lightly floured surface and pat into rectangle. Spread cream cheese filling evenly across surface. Roll dough tightly jelly-roll style and pinch seam firmly to seal.
07 - Shape into round or oval loaf. Place seam-side up in floured proofing basket. Cover and refrigerate for 6 to 10 hours for cold proof.
08 - Position oven rack at center. Place Dutch oven inside and preheat to 450°F for at least 30 minutes to ensure thorough heating.
09 - Turn dough onto parchment paper. Score top of loaf with sharp knife or razor. Sprinkle with coarse sugar and extra lemon zest if using.
10 - Transfer loaf with parchment into preheated Dutch oven. Cover with lid and bake for 25 minutes to create steam and oven spring.
11 - Remove Dutch oven lid and continue baking for 20 minutes until loaf turns deep golden brown and develops crispy crust.
12 - Remove bread from oven and transfer to wire rack. Cool completely before slicing to prevent filling from melting out.