1. In a large mixing bowl, gently whisk together the active sourdough starter and warm water until the starter is mostly dissolved. The mixture should look milky and slightly bubbly.

2. Add the bread flour, salt, and granulated sugar to the wet mixture. Use a sturdy spoon or your hands to mix until all the flour is incorporated and no dry spots remain. The dough will be shaggy and sticky. Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes (autolyse). This step allows the flour to fully hydrate.

3. After the rest, gently fold in the lemon zest. In a separate small bowl, toss the blueberries with 1 tablespoon of all-purpose flour. This dusting helps prevent the blueberries from sinking to the bottom of the loaf. Gently fold the floured blueberries into the dough until evenly distributed. Be careful not to overmix, as this can crush the berries.

4. Over the next 2-3 hours, perform a series of “stretch and folds.” Every 30-45 minutes, lightly wet your hands, grab a portion of the dough from the edge of the bowl, stretch it upwards, and fold it over itself towards the center. Rotate the bowl and repeat this 3-4 times until the dough feels stronger and more elastic. Cover the bowl each time.

5. After the stretch and folds, cover the bowl and let the dough bulk ferment at room temperature for 4-8 hours, or until it has visibly increased in volume by about 30-50% and feels airy. Alternatively, you can place it in the refrigerator for 8-12 hours for a slower, colder fermentation, which can develop more complex flavors.

6. Gently turn the dough out onto a lightly floured surface. Carefully shape it into a round or oval, being careful not to degas it too much. Place the shaped dough seam-side up into a floured banneton or a bowl lined with a floured kitchen towel.

7. Cover the banneton/bowl and let the dough proof at room temperature for 1-2 hours, or until it looks puffy and springs back slowly when gently poked. Alternatively, you can refrigerate it for a cold proof for another 2-4 hours, or even overnight, which can make it easier to score.

8. Preheat your oven to 450 F (230 C) with a Dutch oven inside for at least 30 minutes. Carefully remove the hot Dutch oven, invert the dough into it, score the top with a sharp knife or lame, and bake covered for 30 minutes. Remove the lid, reduce the temperature to 400 F (200 C), and bake for another 15-25 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210 F (93-99 C).

9. Transfer the baked bread to a wire rack and let it cool completely for at least 1-2 hours before slicing. This is crucial for the crumb structure and flavor to set properly.