Dissolve horseradish and mustard in a few tablespoons of kefir and let steep while you prepare the vegetables.
In a mortar and pestle, pound the spring onions, dill, coriander, and parsley with a few generous pinches of salt until aromatic and bruised (not a paste - just a few good bashes).
In a large bowl, combine the cubed potatoes with the herb mixture, mixing roughly to work the herbs through.
Stir in the kefir-horseradish-mustard mixture.
Gently fold in the eggs, cucumber, and radish. Season with salt to taste.
Chill for at least 1 hour to let flavours meld.
Pile the vegetable mixture into shallow bowls. Serve alongside a jug of the kefir-whey mixture for guests to pour over themselves, plus small dishes of Georgian Salt, extra dill, cornichons, and any other condiments you fancy.
Variation: For a simpler approach, combine everything together in one pot before chilling, then serve as a traditional soup.
