Prepare your sourdough starter as in Tartine country bread instruction. You can also use your own 100% hydration old starter that has been sitting in the fridge; it will happily revive.
The whole process happens over 4 days: making leaven on the night of day 1; working the dough on day 2; shaping the loaf on day 3 and baking on the morning of day 4. There’s absolutely nothing wrong with baking on day 3, having allowed the loaf to prove a couple of hours in a warm place, but nothing beats fresh bread in the morning, does it?
1. The night of day 1 prepare the leaven: discard all but 1 tbsp. of the starter. Mix it with 200g warm water until dispersed. Add the flour, stir it until combined and leave at room temperature for 12 hours. It should become bubbly and puffed up. To test if it’s ready, scoop a teaspoon of it and see if it floats in a bowl of water. If it sinks, let it mature longer.
2. For the main dough, mix 125g of the leaven (the rest will become your starter for future baking and can live in the fridge) with 350g warm water in a large bowl; stir to disperse. Add the 400g white flour and the 100g wholemeal and mix to a rough dough with your hands or a dough whisk until there is no more dry flour visible. Cover the bowl with a damp tea towel and leave to rest for 25-40 minutes at room temperature.
3. Add the salt and the remaining 25g of warm water to the dough and mix with your hands, the dough whisk or in a standing mixer with a dough hook attachment until it smooths a little and starts pulling away from the sides of the bowl. Cover with the damp tea towel and leave in a warm place for 30 minutes.
4. Toast the walnuts, broken into small chunks, in the oven at 200C/400F/gas 6 for about 10 minutes; until a little darker and fragrant. Leave to cool.
5. For the next 3 hours stretch and fold the dough every 30 minutes, then return it to the warm place. To do that, wet your hands; grab the underside of the dough at one quadrant and stretch it up over the rest of the dough. Repeat this three more times, rotating bowl a quarter turn for each fold. Do this every half an hour, six times in total. Add the walnuts at the second turn, pressing them into the dough to incorporate evenly.
6. At the end of the session the dough should increase in volume 20 to 30 percent. There will be some black streaks going through it from the tannins that the walnuts contain. That’s nothing to worry about and will look quite attractive in the baked loaf. Cover the bowl with cling film and place in the fridge for up to 24 hours.
7. The next day take it out early, so it returns to room temperature; it will take a couple of hours. When it gets warmer, turn it out onto a work surface and dust the top with flour. Flip it floured side down and fold it over, so that the outside is all floured; form into a round. Dust with more flour, cover with a towel and leave for 30 minutes.
8. Prepare a proving basket or bowl lined with cloth flouring it generously with the whole-rice flour mix.
9. Dust the dough round with flour and shape to an oblong loaf (here’s how). Transfer the loaf into the basket seam side up. Cover with a towel and return to the fridge for the next 18 – 20 hours.
10. The next morning preheat a pizza stone or a heavy baking sheet for at least 40 minutes in the oven at 250C/500F/max gas. Dust the top of dough, still in the basket, with the wholemeal/rice-flour mixture. Prepare a length of parchment to turn the loaf out onto. Do it carefully, not to deflate the loaf. Slash along the top with a baker’s razor or a sharp knife.
11. Transfer the loaf with the parchment into the oven using a peel or another flat baking sheet. Spray the oven with water or drop a very wet cloth on the bottom of the oven. Reduce temperature to 230C/450F and bake for 50 minutes. Reduce the temperature again after 40 minutes to 200C/400F/gas 6 if the loaf is getting too dark. Transfer bread to a wire rack to cool for at least 15 minutes before slicing.