In a large bowl, combine the rye flour, wheat flour, water, salt, rye sourdough starter, and walnuts. Mix until no dry flour remains.
Cover the bowl and let the dough rest at room temperature for 24 hours. During the first 12 hours, gently stretch and fold the dough once or twice if desired.
After about 12 hours, transfer the dough to a lightly floured surface and gently shape it into a round loaf.
Place the shaped dough into a floured proofing basket or bowl lined with a clean kitchen towel.
Let the dough rest for another hour at room temperature.
About 30 minutes before baking, preheat the oven and a Dutch oven or heavy baking pot to 250°C (480°F).
Carefully transfer the dough into the hot pot, cover with the lid, and bake for 30 minutes.
Remove the lid, reduce the temperature to 230°C (445°F), and bake for another 15–18 minutes until deeply golden and crisp.
Transfer the bread to a wire rack and allow it to cool completely before slicing.