Cook potatoes in a large pot of boiling salted water over medium heat just till tender, 15-20 min.

Meanwhile, make balsamic dressing: Whisk lowfat sour cream, balsamic vinegar, extra virgin olive oil, salt and pepper in a large bowl till blended.

Stir in all but 2 tb sliced scallions.

Drain potatoes well in a colander.

Transfer to bow; add in dressing and gently toss to coat.

Let potatoes cold to room temperature.

Add in 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.