Refresh the sourdough starter (I used a starter at 100% hydration with equal amounts of rye and bread flour).
Toast the walnuts in an oven heated to 200°C for a few minutes. Then cool the walnuts and leave to soak in cold water overnight.
Mix the flours and water and let them stand in the mixing bowl for 1 hour. This autolyse step hydrates the flours and gives gluten forming proteins a good start at building the gluten network.
After the autolyse period, knead the dough for 10 minutes on the table. Then add salt and knead for a further 5 minutes.
Place the dough back into the (lightly floured) bowl and leave to rest for 30 minutes.
Drain the walnuts.
Take the dough out of the bowl, stretch it out on the table and sprinkle the walnuts on top. Fold carefully a few times to distribute the walnuts evenly in the dough. Shape the dough into a ball and put it back in the lightly floured bowl.
When the dough has roughly doubled in size (about 2-3 hours later depending on temperature and the activity of your starter), divide the dough in three pieces.
Shape each piece into a round loaf and place in a proving basket.
Leave the breads on the kitchen counter for a final proof, or do as I did and place them in the refrigerator to slow down to fermentation and build a sweeter and more interesting flavour.
Pre-heat the oven to 250°C.
Bake the breads in a dutch oven or on a baking stone at 230°C for 45-50 minutes. If you use a dutch oven, remove the lid after 25 minutes in order to get a good, dark crust.