01 - Combine bread flour, whole wheat flour, sourdough starter, and water in a large mixing bowl. Mix thoroughly until no dry flour remains visible. Cover the bowl and let autolyse for 45 minutes.
02 - Add fine sea salt to the dough and knead briefly until fully incorporated. Cover and allow the dough to rest for 30 minutes.
03 - Complete 3 to 4 sets of stretch-and-fold technique every 30 minutes during the initial 2 hours of bulk fermentation to develop gluten structure.
04 - While the dough ferments, beat softened cream cheese with granulated sugar and vanilla extract until completely smooth and well combined.
05 - Gently fold fresh blueberries and lemon zest into the dough during the final stretch-and-fold set, distributing evenly throughout.
06 - Turn the dough onto a lightly floured surface and pat into a rectangular shape. Spread the cream cheese mixture evenly across the surface. Roll the dough tightly jelly-roll style and firmly pinch the seams to seal.
07 - Form the dough into a tight round or oval shape. Place seam-side up in a floured proofing basket. Cover and refrigerate for 6 to 10 hours for cold overnight proofing.
08 - Preheat oven to 450°F with the Dutch oven inside for at least 30 minutes to ensure proper heat retention.
09 - Carefully turn the proofed dough onto parchment paper. Score the top of the loaf with a sharp knife or lame. Sprinkle with coarse sugar and additional lemon zest if desired.
10 - Transfer the loaf with parchment into the preheated Dutch oven. Bake covered for 25 minutes to create steam and oven spring.
11 - Remove the Dutch oven lid and continue baking for 20 additional minutes until the crust develops a deep golden color and sounds hollow when tapped.
12 - Transfer the baked loaf to a wire cooling rack. Allow to cool completely before slicing to prevent the cream cheese filling from becoming runny.