Balsamic Potato Salad
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.
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.