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 with a damp towel and let autolyse for 45 minutes.
02 - Add fine sea salt to the dough and knead briefly to distribute evenly. Cover and allow the dough to rest for 30 minutes.
03 - Perform 3 to 4 sets of stretch-and-fold techniques at 30-minute intervals throughout the first 2 hours of bulk fermentation. This develops gluten structure and strength.
04 - While dough ferments, beat softened cream cheese, granulated sugar, and vanilla extract in a small bowl until smooth and fully combined.
05 - During the final stretch-and-fold, gently fold fresh blueberries and lemon zest into the dough, being careful not to crush the berries.
06 - Turn dough onto a lightly floured work surface and pat into a rectangle. Spread cream cheese mixture evenly over surface. Roll dough tightly jelly-roll style and pinch seams 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 (overnight) for cold proofing.
08 - Place Dutch oven or lidded cast iron pot in oven and preheat to 450°F for at least 30 minutes to ensure proper heat retention.
09 - Turn dough onto parchment paper, score the top with a sharp knife or lame, and sprinkle with coarse sugar and extra lemon zest if using. Transfer to preheated Dutch oven, cover with lid, and bake for 25 minutes.
10 - Remove lid and continue baking for 20 minutes until deeply golden and crusty. Internal temperature should reach 200°F.
11 - Transfer loaf to wire rack and cool completely before slicing to prevent gumminess. Serve toasted with additional cream cheese or lemon curd if desired.