Mix Dough: In a large bowl, combine sourdough starter and warm water until dissolved. Add whole wheat flour, bread flour, oats, sunflower seeds, flaxseeds, chia seeds, and salt. Mix until all ingredients are combined into a shaggy dough.
- Autolyse: Cover the bowl and let the dough rest for 30 minutes to hydrate the flour.
- Knead and Bulk Fermentation: Perform a series of stretch and folds every 30 minutes over 2 hours to develop gluten. Then cover and let the dough ferment at room temperature for 4-6 hours until doubled in size.
- Shape the Loaf: Turn dough onto a floured surface, shape into a tight round loaf, and place seam-side up in a well-floured banneton or bowl lined with a towel.
- Final Proof: Cover and let proof for 2-4 hours at room temperature, or refrigerate overnight for a slower fermentation and enhanced flavor.
- Preheat Oven: Place a Dutch oven or heavy oven-safe pot in the oven and preheat to 475°F (245°C).
- Bake: Carefully transfer the dough to the hot Dutch oven, score the top with a sharp blade, cover, and bake for 20 minutes. Remove the lid and bake for another 20-25 minutes until the crust is deeply golden and crisp.
- Cool: Remove the bread from the oven and cool completely on a wire rack before slicing.