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 all-purpose flour, salt, and granulated sugar to the wet mixture. Use a sturdy spoon or your hands to mix until a shaggy dough forms and no dry flour remains. It will be sticky and a bit messy at this stage. Cover the bowl with a clean kitchen towel or plastic wrap and let it rest for 30 minutes (this is called autolyse, and it helps the flour hydrate).

3. After the rest period, uncover the dough. Sprinkle the lemon zest evenly over the dough, then add the blueberries. Gently fold the dough over itself several times to distribute the zest and blueberries throughout. Be careful not to overmix, as this can crush the blueberries.

4. Lightly oil a clean bowl with olive oil or melted butter. Transfer the dough to this bowl, turning it once to coat. Over the next 4-8 hours (or until the dough has visibly increased in size and is bubbly), perform a series of “stretch and folds.” Every 30-60 minutes during the first 2-3 hours, gently stretch a portion of the dough up and fold it over itself towards the center. Rotate the bowl a quarter turn and repeat 3-4 times. This builds strength in the dough.

5. Once the bulk fermentation is complete, gently turn the dough out onto a lightly floured surface. Carefully shape it into a round or oval loaf, being mindful not to deflate it too much. You can do this by gently pulling the edges of the dough towards the center and then flipping it over to create surface tension.

6. Place the shaped dough into a floured banneton basket or a bowl lined with a floured kitchen towel, seam-side up. Cover it loosely with plastic wrap. You can either proof it at room temperature for 1-4 hours (until it passes the “poke test” – springs back slowly) or refrigerate it overnight for 8-12 hours for a deeper flavor and easier handling.

7. Preheat your oven to 450 F (232 C) with a Dutch oven inside for at least 30 minutes. Carefully remove the hot Dutch oven, invert the dough into it, and score the top with a sharp knife or razor blade. Cover and bake for 30 minutes. Remove the lid, reduce the oven temperature to 400 F (200 C), and continue baking for another 15-25 minutes, or until the crust is golden brown and the internal temperature reaches 200-210 F (93-99 C).

8. Transfer the baked loaf to a wire rack and let it cool completely for at least 1-2 hours before slicing. This is crucial for the crumb to set properly and for the flavors to fully develop.