Rinse beet greens thoroughly, cut off the roots. Peel beets and cut into small cubes. Chop stems finely, chop leaves into about 2-3 mm pieces. If beet greens have many leaves, not all need to be added.
Bring stock to a boil, add only the cut beets, reduce heat and cook for about 5 - 7 minutes, then add stems and leaves and cook for about 3 minutes.
Add lemon juice and vinegar and cook for about 1 minute, season with salt and pepper if needed. Cool the stock.
Add kefir to the cooled stock and mix. Chill. Serve with, for example, egg, cucumber, and radish.