For the grain to grain bread, mix the two types of flour together and make a well in the center. Mix the sourdough and yeast with 250 ml of water in a small bowl and pour into the well.
Mix with about half of the flour and let rest, covered, in a warm place for about 12 hours. Soak the wheat and barley grains in water, covered, for 6 hours and then simmer on low heat for 60 minutes.
Soak the oat and buckwheat grains in water for 3 hours and then simmer for 30 minutes. Finally, drain all the grains through a sieve. Knead the pre-dough with salt and the remaining water, as well as the remaining flour, to a smooth dough.
Set aside about half of the sunflower seeds, knead all the other grains and seeds into the dough. Pour the dough into 2 loaf pans and let rise, covered, for another 2 hours.
Then brush the surface of the dough with water and spread the reserved sunflower seeds on top. Place an ovenproof dish with water in the oven preheated to 220 °C, add the bread and immediately close the oven.
Bake the bread for 40 minutes, then turn the temperature down to 200 °C and bake for another 30 minutes. Take out the finished loaves, remove from the pan and let the grain to grain bread cool on a cooling rack.