Wash the beets but leave the 'tail' attached.
Boil rapidly for 45 minutes, until tender, then peel and cut into long, thin strips.
Place the strips in a mixing bowl.
Trim and thinly slice the scallions and add them to the beets.
Mix together the sour cream, mustard, lemon juice and seasoning in a cup, and pour it over the onion and beet mixture.
Stir well and serve immediately or allow to chill.