In separate saucepans, cook beans and corn in lightly salted boiling water until tender-crisp (the beans for 4-5 minutes, the corn for 2-3 minutes). Drain, then rinse thoroughly under cold running water. Drain well.
Spread vegetables on clean tea towel and pat dry. Transfer to serving bowl. At this point you may refrigerate, covered, up to 24 hours.
In medium bowl, whisk together vinegar, mustard, honey and gingerroot. Gradually whisk in oil until well combined. Season with salt and pepper to taste. Refrigerate, covered, up to 24 hours.
Just before serving, add dressing and onions to vegetables. Toss well.