1. Whisk together first 7 ingredients until blended. Cover and chill 30 minutes. (Dressing may be stored in refrigerator up to 3 days.)
2. Cut romaine hearts in half lengthwise, keeping leaves intact. Arrange halves on individual serving plates. Sprinkle with radishes. Drizzle with vinaigrette. Season with salt and pepper to taste. Garnish, if desired.