Wash and thinly slice radish, cucumber, and onion.
Put the radish, cucumber, and onion in a pot, add water, and bring to a boil.
Once boiling, add kelp and remove it after 5 minutes.
When the broth is infused, strain out all the vegetables and let it cool.
Season with lemon juice, salt, and vinegar to taste.
Store in the refrigerator until cold.