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 dough autolyse for 45 minutes to develop gluten structure.
02 - Add fine sea salt to the dough and knead briefly to distribute evenly. Cover and allow the dough to rest for 30 minutes before beginning stretch-and-fold sequences.
03 - Perform 3 to 4 sets of stretch-and-folds at 30-minute intervals throughout the first 2 hours of bulk fermentation. This technique develops the dough's structure and creates a strong gluten network.
04 - While the dough ferments, beat softened cream cheese with granulated sugar and vanilla extract until completely smooth and creamy. Set aside until ready to fill.
05 - During the final stretch-and-fold sequence, gently fold fresh blueberries and lemon zest into the dough. Take care to distribute the berries evenly without crushing them.
06 - Turn the dough onto a lightly floured work surface and pat into a rectangular shape. Spread the cream cheese mixture evenly over the surface, then roll the dough jelly-roll style. Pinch the seam firmly to seal.
07 - Shape the filled dough into a tight round or oval. 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 - Position a Dutch oven or lidded cast iron pot in the oven. Preheat to 450°F for at least 30 minutes to ensure the vessel is thoroughly hot for optimal oven spring.
09 - Turn the proofed dough onto a piece of parchment paper. Use a lame or sharp knife to score the top of the loaf. Sprinkle with coarse sugar and additional lemon zest if desired.
10 - Lift the dough with parchment paper into the preheated Dutch oven. Cover with the lid and bake for 25 minutes. The covered baking creates steam for a crispy, crackly crust.
11 - Remove the lid from the Dutch oven and continue baking for 20 minutes until the loaf develops a deep golden-brown crust and sounds hollow when tapped on the bottom.
12 - Transfer the baked loaf to a wire cooling rack. Allow to cool completely for at least 2 hours before slicing to prevent the cream cheese filling from becoming runny and to let the crust fully set.