Mix 50 grams wheat flour with 50 grams of water and 25 grams of sourdough starter. Place the levain at room temperature and let it ferment for 8 hours, or until it has at least doubled in size.

Preheat the oven to 350ºF/175ºC. Spread the walnuts on parchment paper and toast them in the oven for 8 to 10 minutes. Watch them not getting burnt.

Mix flour with water and let the dough autolyse for an hour. Add levain and salt and incorporate them into the dough. Let the dough rest for 15 minutes before kneading the dough until smooth.

Let the dough bulk ferment for 4 hours at a temperature of 79ºF/26ºC, or until it has increased 50-70% in volume. Perform two stretch and folds, spaced out by 30 minutes at the beginning of bulk fermentation. Add the walnuts during the last stretch and fold.

Dump out the dough and preshape it into a round. Let it rest for 15 minutes. Shape the dough into a Batard and place it into a lightly floured banneton.

Let the dough proof at room temperature, 72ºF/22ºC, for 1 - 1.5 hours. Or let it sit at room temperature for 30 minutes and then place it in the refrigerator for 10 hours.

Preheat the oven to 480ºF / 250ºC. Bake for 45-50 minutes on an oven plate or baking stone or a dutch oven. Add some steam if you don't have anything to cover the loaf. You may have to lower the heat to 430ºF / 220ºC after 15-20minutes.

Let the bread cool on wire racks.