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.
02 - Add fine sea salt to the dough and knead briefly until fully incorporated. Cover and let rest for 30 minutes to allow gluten development.
03 - Perform 3 to 4 sets of stretch-and-fold technique at 30-minute intervals during the first 2 hours of bulk fermentation. This develops the gluten structure for proper shaping.
04 - While dough ferments, beat softened cream cheese with granulated sugar and pure vanilla extract until smooth and fully combined. Set aside until ready to use.
05 - During the final stretch-and-fold set, gently fold fresh blueberries and lemon zest into the dough. Handle carefully to distribute evenly without crushing the berries.
06 - Turn dough onto a lightly floured surface and pat into a rectangular shape. Spread the cream cheese mixture evenly over the surface. Roll the dough jelly-roll style and pinch seams firmly to seal.
07 - Shape the filled 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 - Place Dutch oven or lidded cast iron pot in the oven and preheat to 450°F. Allow at least 30 minutes for thorough heating to ensure proper oven spring.
09 - Turn proofed dough onto parchment paper. Score the top with a sharp knife or lame. Sprinkle with coarse sugar and extra lemon zest if using. Transfer to preheated Dutch oven, cover, and bake for 25 minutes.
10 - Remove the lid from the Dutch oven and continue baking for 20 minutes until the crust develops a deep golden color and sounds hollow when tapped on the bottom.
11 - Transfer the baked loaf to a wire cooling rack. Cool completely before slicing to prevent the cream cheese filling from becoming runny and the crust from losing its texture.