Fresh green and yellow beans dressed with a mustard vinaigrette make a tasty side-dish salad for grilled meats. Leave the beans whole for visual impact.
Whisk vinegar and mustard in small bowl. Whisk in oil, salt and pepper.
Steam beans until crisp-tender, 10 minutes. Run beans in colander under cold water to stop cooking; drain.
Combine beans and onion in serving bowl. Add dressing. Let stand at room temperature 30 minutes. For best flavor, refrigerate overnight; serve at room temperature.