Dressing: Whisk first 5 ingredients gradually. Whisk in oil. Season with salt and pepper. (Can be made day before, warmed to room temperature and rewhisked.) Cook green beans until tender, approximately 5 minutes. Put in bowl of ice water to cool. Drain and cut in half. Combine green beans, potatoes, onion and basil in a large bowl. Add dressing; toss to coat. Season with salt and pepper. (Can be prepared 4 hours ahead; cover and let stand at room temperature.) Can be doubled.

Adapted from Bon Appetit, July, 1994.