1. In a large bowl combine bread flour, whole wheat flour, active sourdough starter, and water until a shaggy dough forms and let rest for 30 to 45 minutes for autolyse.

2. Sprinkle in sea salt, sugar, lemon zest, and poppy seeds, then knead or squeeze the dough until all ingredients are evenly distributed.

3. Cover the dough and bulk ferment at room temperature for 4 to 6 hours, performing a set of stretch and folds every 30 minutes during the first 2 hours.

4. Turn the dough onto a lightly floured surface, shape into a round or oval loaf, and place seam-side up in a floured proofing basket.

5. Cover and refrigerate the shaped loaf for 8 to 12 hours for cold proofing.

6. Preheat the oven to 475°F (245°C) with a Dutch oven and lid inside for at least 30 minutes.

7. Use a silicone bread sling to lift the dough into the hot Dutch oven, score the top with a sharp blade, and cover.

8. Bake covered for 20 minutes, then remove the lid and bake for an additional 20 to 25 minutes until the crust is deeply golden.

9. Remove the loaf and cool on a wire rack for at least 1 hour before slicing.