Soak the bread yeast in 4 liters of cold water. Allow to stand for at least 1 hour. But it's best if you remember and soak it overnight. Soak the dried mushrooms in a cup of water for 10 minutes to soften them. Wash the potatoes and cook them in their skins. Peel the boiled soft potatoes and cut them into larger cubes. Dissolve the pre-soaked yeast in water, add salt, cumin, mushrooms and cook for almost an hour, until the yeast turns vitrified and gradually thickens. Stir while cooking. Peel and finely chop the onion. Fry it in butter until golden. Add the raw eggs to the onion and mix lightly. We do not fry the eggs, they should remain partially raw. When the water with the yeast is no longer cloudy, it is thick and has a glassy color, briefly stir the onion and eggs into it and cook. The eggs will collide. All you have to do is add boiled, cubed potatoes to the Krkonoše sour and we can serve the sour.

Kyselo or Krkonošské kyselo is a soup made from bread kvass, especially widespread in the Krkonoše and Podorlicka regions. The first verified evidence of acid dates back to 1803.[1]