01 - In a large bowl, combine bread flour, whole wheat flour, sourdough starter, and water. Mix thoroughly until no dry flour remains visible. 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 distributed. Cover 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 initial 2 hours of bulk fermentation. Grab one edge of the dough, stretch it upward, and fold it over the center. Rotate the bowl and repeat for all four sides.
04 - While dough ferments, beat softened cream cheese, granulated sugar, and vanilla extract in a small bowl until completely smooth and creamy.
05 - During the final stretch-and-fold cycle, gently incorporate fresh blueberries and lemon zest into the dough. Handle carefully to avoid crushing the berries.
06 - Turn dough onto a lightly floured work surface. Pat into a rectangular shape. Spread cream cheese mixture evenly across the surface, leaving a small border. Roll the dough tightly jelly-roll style, pinching the seam firmly to seal.
07 - Shape into a tight round or oval loaf. Place seam-side up in a floured proofing basket or bowl lined with a floured towel. Cover and refrigerate for 6 to 10 hours for overnight cold proofing.
08 - Place Dutch oven or lidded cast iron pot in cold oven. Preheat to 450°F for at least 30 minutes to ensure thorough heating.
09 - Turn dough out onto parchment paper. Score the top with a sharp knife or lame. Sprinkle with coarse sugar and extra lemon zest if desired. Transfer to preheated Dutch oven, cover with lid, and bake for 25 minutes.
10 - Remove the lid and continue baking for 20 minutes until the loaf develops a deep golden crust and sounds hollow when tapped on the bottom.
11 - Transfer bread to a wire rack and allow to cool completely before slicing to prevent the filling from becoming runny. Serve toasted with additional cream cheese or lemon curd if desired.