1. 1. Combine Wet Ingredients: In a large mixing bowl, combine your active sourdough starter (or discard), warm water, granulated sugar, and salt. Whisk these ingredients together until the sugar and salt are dissolved and the mixture is well combined. It should look like a cloudy, slightly bubbly liquid.

2. 2. Add Flour and Mix Dough: Gradually add the all-purpose flour to the wet ingredients, mixing with a wooden spoon or your hands until a shaggy dough forms. Once all the flour is incorporated, turn the dough out onto a lightly floured surface and knead for about 5-7 minutes until it becomes smooth and elastic. It should spring back slightly when poked.

3. 3. Incorporate Blueberries and Lemon: Flatten the dough into a rough rectangle. Sprinkle the fresh or frozen blueberries, lemon zest, and fresh lemon juice evenly over the surface of the dough. Gently fold the dough over itself and continue to knead for another 2-3 minutes, just until the blueberries and lemon are evenly distributed throughout the dough. Be careful not to overmix, as this can crush the blueberries too much.

4. 4. First Rise (Bulk Fermentation): Place the dough into a lightly oiled bowl, turning it once to coat. Cover the bowl with plastic wrap or a clean kitchen towel and let it rise in a warm spot for 2-3 hours, or until it has visibly increased in size (about 50-75%). Alternatively, you can place it in the refrigerator overnight for a slower, colder proof.

5. 5. Shape the Loaf: Gently turn the risen dough out onto a lightly floured surface. Carefully shape it into a round or oval loaf. Try to maintain as much of the air in the dough as possible. If you have a proofing basket (banneton), dust it with flour and place the shaped dough seam-side up. If not, place it on a parchment-lined baking sheet.

6. 6. Second Rise (Final Proof): Cover the shaped loaf loosely with plastic wrap or a kitchen towel and let it proof for another 30-60 minutes at room temperature, or until it looks puffy. If you did an overnight cold proof, you can bake it straight from the fridge.

7. 7. Preheat Oven and Bake: While the loaf is doing its second rise, preheat your oven to 425 F (220 C). If using a Dutch oven, place it in the oven while it preheats. Once preheated, carefully transfer the loaf to the hot Dutch oven (or directly onto the baking sheet). Score the top of the loaf with a sharp knife or razor blade (a simple cross or a few parallel lines work well).

8. 8. Bake Until Golden: Bake for 30 minutes with the lid on (if using a Dutch oven), then remove the lid and bake for another 10-15 minutes, or until the crust is golden brown and the internal temperature reaches 200-210 F (93-99 C). Let the bread cool completely on a wire rack before slicing and serving. This is crucial for the texture to set properly.