Cut potatoes into 3/4-inch wedges. On a large steamer rack set over boiling water steam potatoes, covered, until just tender, about 10 minutes, and transfer to a bowl. Steam beans on rack over boiling water, covered, until just tender, about 6 minutes, and transfer to bowl. Vegetables may be steamed 1 day ahead and chilled, covered. Bring vegetables to room temperature before proceeding.
In a large bowl whisk together miso, citrus juices, oil, and vinegar and add steamed vegetables, parsley, and peperoncini, tossing to combine well. Season salad with salt if necessary.