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 mixture autolyse for 45 minutes to develop gluten structure.
02 - Add sea salt to the dough and knead briefly until fully distributed. Cover and allow the dough to rest for 30 minutes before beginning stretch-and-fold sequence.
03 - Perform 3 to 4 sets of stretch-and-fold techniques at 30-minute intervals throughout the first 2 hours of bulk fermentation. Grab one edge of the dough, stretch it upward, and fold it toward the center. Rotate the bowl and repeat for all four sides.
04 - While dough ferments, beat softened cream cheese with granulated sugar and vanilla extract until smooth and creamy. Set aside at room temperature for easy spreading.
05 - During the final stretch-and-fold set, gently incorporate fresh blueberries and lemon zest into the dough. Use light folding motions to distribute evenly without crushing the berries.
06 - Turn the dough onto a lightly floured work surface. Pat into a rectangular shape. Spread the cream cheese mixture evenly across the surface. Roll the dough tightly like a jelly roll, pinching the seam firmly to seal.
07 - Shape the rolled dough into a 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 overnight cold proofing.
08 - Position a Dutch oven or lidded cast iron pot on the center rack. Preheat oven to 450°F for at least 30 minutes to ensure thorough heating of the cooking vessel.
09 - Turn the proofed dough onto a piece of parchment paper. Use a sharp knife or lame to score the top of the loaf. Sprinkle with coarse sugar and additional lemon zest if desired.
10 - Lift the dough with parchment into the preheated Dutch oven. Cover with the lid and bake for 25 minutes to create steam and develop oven spring.
11 - Remove the lid from the Dutch oven. Continue baking for an additional 20 minutes until the loaf develops a deep golden 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 gummy. Slice thickly and serve toasted with additional cream cheese or lemon curd.