In a large mixing bowl, combine your bubbly active sourdough starter with warm water, bread flour, and all-purpose flour. Stir until just combined, creating a shaggy dough. Cover the bowl with a damp cloth and let it rest for about 30 minutes.

After the rest, sprinkle salt over the dough and knead it on a lightly floured surface for about 8-10 minutes until smooth and elastic.

Transfer the kneaded dough back to the bowl and cover it with a clean cloth. Allow it to rise at room temperature for 3-4 hours, stretching and folding the dough every hour.

Once your dough has risen, gently deflate it on a floured surface and sprinkle in the fresh blueberries and lemon zest. Carefully fold the ingredients into the dough.

Shape the dough into a rectangle on a floured surface, about 1-inch thick. Spread your sweetened cream cheese filling over the surface, leaving a small border, then roll the dough tightly into a log.

Place the shaped loaf into a floured banneton or loaf pan, cover it with a cotton cloth, and refrigerate overnight.

The next day, preheat your oven to 450°F (230°C) with a Dutch oven inside. Score the top of the dough with a sharp knife and bake covered for 20 minutes, then remove the lid and bake for an additional 20-25 minutes.

Once baked, transfer the loaf to a wire rack and allow it to cool completely.