Try watching videos below (they are in Polish but 3rd video has CC translated). This type of bread is Polish Lithuanian tradition. You just mix flour with water and keep it warm for a few days, let it ferment to make 'zakwas' - sourdough.

You use rye flour, possibly crude. I have purchased type 720 rye flour (pytlowa - Google translates that as bolted?).

My Mom then experimented and fine-tuned these recipes.

https://youtu.be/6BeDDl8-VD0

https://youtu.be/PzWD89vITNQ

https://youtu.be/qhq32b7zMwE

This is a link for this particular flour on Polish auction site, they are shipping to Europe.

https://allegro.pl/oferta/maka-zytnia-typ-720-10kg-chlebowa-...