Cook potatoes in a large pot of boiling salted water over medium heat just until tender, 15-20 minutes.
Meanwhile, make balsamic dressing: Whisk sour cream, balsamic vinegar, olive oil, salt and pepper in a large bowl until blended. Stir in all but 2 tb sliced scallions. Drain potatoes well in a colander.
Transfer to bowl; add dressing and gently toss to coat. Let potatoes cool to room temperature. Add celery and radishes to potatoes in bowl and toss gently to combine. Line serving bowl with romaine leaves and spoon potato salad onto lettuce. Sprinkle salad with reserved scallions. 105 calories per serving.
From: Syd's Cookbook.