In a large mixing bowl, combine the bread flour and whole wheat flour with 350g warm water. Mix until no dry spots remain and a shaggy dough forms. Cover the bowl with a clean kitchen towel or plastic wrap and let it rest for 30 minutes to 1 hour (autolyse). This step hydrates the flour and begins gluten development.

After the autolyse, add the active sourdough starter and salt to the dough. Incorporate thoroughly by squeezing and folding the dough with your hands for 3-5 minutes until the starter and salt are evenly distributed. The dough will be sticky at this stage.

Cover the bowl again and let the dough rest for 30 minutes. After 30 minutes, perform your first set of 'stretch and folds'. Gently pull a section of the dough up from the edge of the bowl, stretch it, and fold it over itself towards the center. Rotate the bowl and repeat this process 3-4 times until you've worked your way around the dough. This builds strength.

Sprinkle the blueberries, lemon zest, and optional sugar over the dough. Gently perform another set of stretch and folds, trying to evenly distribute the inclusions without crushing the blueberries too much. Be gentle.

Repeat the stretch and fold process every 30-45 minutes for a total of 4-5 sets over the next 3-4 hours, or until the dough has become noticeably puffy, smooth, and has increased in volume by about 30-50%. The total bulk fermentation time will vary depending on your kitchen temperature and starter activity. Aim for a jiggly, active dough.

Lightly flour your work surface. Gently turn the dough out onto the surface. Using a dough scraper and your hands, gently form the dough into a loose round (boule). Avoid degassing it too much. Let the dough rest, uncovered, for 20-30 minutes (bench rest).

Prepare your proofing basket (banneton) by dusting it generously with rice flour (or all-purpose flour) to prevent sticking. After the bench rest, gently tighten the dough into its final shape. For a round loaf, gently fold the edges of the dough into the center, flipping it seam-side up, then rotate and pull towards you to create surface tension. Transfer the shaped dough, seam-side up, into the prepared banneton.

Cover the banneton with a plastic bag or shower cap and place it in the refrigerator for 12-18 hours (cold proof). This slows down fermentation, develops flavor, and makes the dough easier to handle and score.

About 45-60 minutes before you're ready to bake, preheat your oven to 475°F (245°C) with a Dutch oven inside. Allow the Dutch oven to heat thoroughly.

Once the oven and Dutch oven are fully preheated, carefully remove the hot Dutch oven. Gently invert the cold dough from the banneton onto a piece of parchment paper. Using a sharp razor blade or lame, score the top of the dough with your desired pattern. A simple cross or a single long slash works well.

Carefully lower the dough (on the parchment paper) into the hot Dutch oven. Place the lid back on and bake for 25 minutes.

After 25 minutes, remove the lid from the Dutch oven and continue baking for another 20-25 minutes, or until the crust is a deep golden brown and the internal temperature reaches 200-210°F (93-99°C). The total bake time for your blueberry lemon sourdough bread is approximately 45-50 minutes.

Carefully remove the bread from the Dutch oven and transfer it to a wire rack to cool completely for at least 2-3 hours before slicing. Slicing too early can result in a gummy texture.