1. In a large bowl, mix bread flour, whole wheat flour, protein powder, and water until no dry spots remain. Cover and rest for 30 minutes (autolyse).

2. Add the sourdough starter and mix well. Sprinkle in salt and knead until incorporated.

3. During the first 2 hours of bulk fermentation, perform 3–4 sets of stretch and folds every 30 minutes. Add sunflower, chia, and flaxseeds during the second fold.

4. Let dough bulk ferment at room temperature (around 75°F/24°C) for 4–5 hours, or until it has risen 50% and feels airy.

5. Turn dough onto a lightly floured surface and pre-shape into a round. Rest for 20 minutes.

6. Shape into a boule or batard and place seam-side up in a floured proofing basket.

7. Cover and refrigerate overnight (8–12 hours).

8. Preheat oven to 475°F (245°C) with a Dutch oven inside.

9. Transfer dough to parchment paper, score the top, and place into the hot Dutch oven.

10. Bake covered for 20 minutes, then uncover and bake for another 20–25 minutes until deep golden brown.

11. Cool on a wire rack for at least 1 hour before slicing.