Rinse the beets. If they are not already cooked, cook them in water or steam them with a pressure cooker. Rinse them in cold water to cool them down, then peel and cut them into large pieces.
Peel the cucumber (optional) and cut it into large pieces. Chop the spring onions.
In a large blender, mix the vegetables with the kefir (add it gradually).
Add the dill, salt and pepper and taste to adjust the seasoning. The consistency should be rather liquid, if the soup seems too thick, add a little kefir or fermented milk, or water.
For a traditional version, serve with a dash of sour cream as well as hot potatoes (also served with sour cream and dill) and hard-boiled eggs (place 1/2 cut hard-boiled egg in each bowl of soup). You can also serve with homemade potato patties, or gnocchi (for a less traditional version).