It's called "jajka po tatarsku" or "eggs tartar". I don't have the exact recipe but I think you just mix equal parts of mayo and sour cream, finely minced onions and cornishons (little pickles) pepper and salt (not too much salt since mayo is already salty). You may also add pickled mushrooms or peas. Cut hard boiled eggs in half, pour the tartar sauce on them and sprinkle the whole dish with sweet paprika. If you want it to be more creamy, mix one part mayo to two parts sour cream. Enjoy!