Mix 500ml cold, low-calcium water with 100g whole wheat spelt flour and 400g wheat flour type 405 in a mixing bowl.
Mix everything well until a slimy consistency is achieved and the flour is completely bound by the water.
Add 3g dry yeast (or 9g fresh yeast) to the flour-water mixture.
Cover the bowl with a damp kitchen towel and let it sit in the fridge overnight or for 12 hours.