Here is a very simple recipe:

Ingredients:

- Spoiled milk (for sourdough/starter)
- Rye flour
- Warm water

Sourdough/starter:

1st day: 1 cup sour milk + 1 cup rye flour (put in a warm place for 2 days)
3d day: supplement - add 1 cup of milk powder + 1 glass of rye flour (and leave for 2 more days)

Note:
- Put the remaining sourdough/starter  in the refrigerator (cover the jar with a dense gauze, so that the sourdough would breathe).
- When you need to bake again - get the sourdough/starter one day earlier to feed and activate.
- For sourdough, instead of spoiled milk it is possible to use a warm water (did not personally try it)

Mix everything together, place on cooking tray, create a form of the dough, make a slits on top of the dough (for better baking through) and bake in the oven on 400°F (250°C) for a 40-50 min. or until not sticking to the toothpick.

Note: for crispy coat slightly sprinkle with olive oil.