Prepare your sourdough starter as in Tartine country bread instruction. You can also use old starter that has been sitting in the fridge; it will happily revive. The whole process happens over 3 days: making leaven on the night of day 1; working the dough on day 2; shaping the loaf and baking on day 3.
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 200g warm water and the yoghurt in a large bowl; stir to disperse. Add the sugar, the 400g white flour and the 100g rye 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 50g 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. Let it rest for 10 minutes, then knead or mix in the kibbled grain and the linseed. Cover with the damp tea towel and leave in a warm place for 30 minutes.
4. 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. At the end of the session the dough should increase in volume 20 to 30 percent. Cover the bowl with cling film and place in the fridge for up to 24 hours.
5. 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.
6. Prepare a proving basket or bowl lined with cloth flouring it generously with the wholemeal flour.
7. Dust the dough round with flour and shape to a round loaf (here’s how). Transfer the loaf into the basket seam side up. Cover with a towel and leave to prove in a warm place for about an hour.
8. In the meantime 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 some wholemeal flour. Prepare a length of parchment to turn the loaf out onto. Do it carefully, not to deflate the loaf.
9. Slash along the top with a baker’s razor or a sharp knife. 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.
10. Transfer bread to a wire rack to cool for at least 15 minutes before slicing.