Tip: Fold chopped leaves or whole baby leaves int bean, grain and potato salads.
- Whisk together vinegar and thyme in small bowl. Whisk in oil and mustard, and season with salt and pepper, if desired. Stir in chopped onion. Marinate 20 minutes, tossing often.
- Meanwhile, steam potatoes 13 to 14 minutes, or until tender. Transfer steamer rack to work surface, and let potatoes cool 5 minutes.
- Toss potatoes with dressing in wide, shallow bowl. Cool 10 minutes, or until dressing is absorbed, tossing occasionally.
- Steam celery 6 minutes, or until only slight crunch remains. Cool briefly. Fold into potatoes. Fold in mustard greens. Season with salt and pepper, if desired.