Dilute the sourdough with a bit of lukewarm water and let rise for several hours, preferably overnight. Put mushroom and caraway in the pot, add peeled, washed potatoes cut in cubes. Pour in the water, salt and cook until tender. Then add the fermented sourdough mixture and cook. Cut the onion in small pieces and fry on butter or lard, adding it in the soup. Prepare scrambled eggs (the mixture should not be too thick), serving them either directly on the plates or putting them in the soup.
Side dish:
In some regions kyselo is eaten with skin-boiled potatoes, in other regions potatoes are boiled separately, then being cut in cubes and added in the soup.