Activate the leaven as above. For this recipe you want the leaven to be over-fermented, so leave it for at least 72 hours.
Combine the water and cracked rye together and simmer for 30–40 minutes. You should be left with a soft rye porridge. If there is still a little bite in the rye seeds then add a little more water and continue cooking. Take off the heat and chill.
Preheat the oven to 170°C, spread the seeds on a baking tray and toast for 10–15 minutes. They should be rich and dark in colour. Transfer to a bowl and cover with the water. Combine the soaked seeds with the leaven, buttermilk and 150 g of the chilled rye porridge. Mix in the flours and salt, making sure they’re fully incorporated, then cover and leave to ferment for 1 hour. Refrigerate overnight. The next morning line a standard loaf tin with parchment paper. Shape the rye dough to fit inside the tin and leave to prove for 3–4 hours. Preheat the oven to 185°C then bake for 40 minutes.
Remove from the tin and bake directly on the oven shelf for a further 15 minutes. Use a temperature probe to test the core temperature – 95°C and above, and it is ready to eat.