Bring a pot of water to a boil. Add the potatoes to boil for 20-25 minutes, until tender. Halfway through add the eggs to boil for 10 minutes. Transfer potatoes and eggs to an ice water bath to cool. Dice.
Divide all cut ingredients evenly between 4 bowls. Top each with kvass or kefir and a few ice cubes. Season with salt to taste.
Notes
Ingredients can be cooked and cut ahead of time and stored in the refrigerator. When ready to eat, add the soup liquid to the cut ingredients.