In a small bowl, whisk together all of the dressing ingredients until blended. Pour over the salad, tossing to coat.
Notes
I actually prefer this salad without the onions. The slightly peppery flavor of arugula is perfect with just the roasted red peppers, beans and the simple dressing.
Use regular balsamic vinegar if you'd like, but the white variety is nice because it won't make your salad look muddy.