To prepare the soaker, combine the cereal mix, flax seed, and sunflower seeds with the water in a plastic tub. Soak at 75°F/24°C until the soaker has absorbed the water and is slightly dry, 8 to 12 hours.
To prepare the final dough, combine the flours and yeast. Add the sourdough, water, soaker, and salt to the mixer, then add the flours and yeast. Mix with the