To begin, wash and finely dice cucumber, radish, and spring onions. Aim for uniform size for balanced texture in each spoonful. Move to handling the cooked elements.

Boil the potatoes in their skins until fork tender. Cool slightly, peel, and dice evenly. Set aside to cool fully before mixing to avoid warming the soup base. Proceed to prepare the eggs.

Boil the eggs for about 9 minutes until fully set. Cool under cold water, peel, and chop roughly. Transition to chopping the meat.

Dice the cooked beef or smoked sausage into small even cubes. If using leftover roast or ham, trim excess fat for a cleaner broth. Combine with other fillings next.

In a large bowl, combine diced potatoes, eggs, cucumber, radish, spring onion, meat, and herbs. Gently mix to distribute evenly. Prepare the liquid base separately.

In a jug, whisk together kvass or kefir, cold water, sour cream, and vinegar or lemon juice if using. Add salt to taste. Chill if not already cold. Move to final assembly.

Pour the cold liquid base over the mixed solids. Stir gently to combine, ensuring all ingredients are evenly coated and suspended in the broth. Adjust seasoning if needed. Chill before serving.

Cover the bowl and refrigerate for at least 1 hour before serving. This resting period allows the flavours to meld and the soup to chill thoroughly. Prepare garnishes in the meantime.

Ladle the cold okroshka into bowls. Garnish with fresh dill or a dollop of sour cream. Serve with dark rye bread on the side for an authentic Russian touch. For added brightness, squeeze a few drops of lemon juice just before eating.