Cut off the tops of the beets, then wrap each in foil and roast until tender when pierced with a paring knife, 45 minutes to 1 hour. Cool the beets slightly, then peel them under cold running water. Cut the beets into 1/2-inch-thick wedges.
Whisk the sour cream, horseradish, vinegar and scallions together in a small bowl with 1/2 teaspoon salt and a generous amount of pepper.
Put the lettuce in a salad bowl and top with the beets. Drizzle the dressing on top. Toss right before serving.