01 - In a large mixing bowl, combine bread flour, whole wheat flour, sourdough starter, and room temperature water. Mix thoroughly until no dry flour remains visible. Cover the bowl and let the mixture autolyse for 45 minutes to develop gluten structure.
02 - Add fine sea salt to the dough and knead briefly to distribute evenly throughout. Cover again and allow the dough to rest for 30 minutes.
03 - Perform 3 to 4 sets of stretch-and-fold techniques at 30-minute intervals during the first 2 hours of bulk fermentation. This process develops gluten strength and improves dough structure.
04 - While the dough undergoes bulk fermentation, beat softened cream cheese, granulated sugar, and vanilla extract together until completely smooth and well combined. Set aside until ready to use.
05 - Gently fold fresh blueberries and lemon zest into the dough during the final stretch-and-fold set. Take care not to crush the berries to prevent excessive color bleeding.
06 - Turn the dough onto a lightly floured work surface. Pat into a rectangular shape. Spread the prepared cream cheese mixture evenly over the surface. Roll the dough tightly in jelly-roll fashion, pinching the seam firmly to seal.
07 - Shape the rolled dough 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 cold proofing.
08 - Preheat oven to 450°F with a Dutch oven or lidded cast iron pot inside. Allow at least 30 minutes for thorough heating to ensure proper oven spring.
09 - Carefully turn the proofed dough onto parchment paper. Score the top of the loaf with a sharp blade. Sprinkle with coarse sugar and additional lemon zest if desired. Bake covered for 25 minutes.
10 - Remove the lid from the Dutch oven and continue baking for an additional 20 minutes until the crust develops a deep golden color and sounds hollow when tapped. Transfer to a wire rack.
11 - Allow the loaf to cool completely on a wire rack before slicing. This ensures proper crumb structure and prevents the filling from becoming too runny. Serve toasted with additional cream cheese or lemon curd if desired.