Steam the potatoes by arranging them in a single layer in a steamer basket over rapidly boiling water. Cover and cook until they are tender, about 15 minutes. As soon as they are cool enough to handle, peel and cut into 1/2 inch slices. Combine potatoes with onions in a mixing bowl.
Meanwhile heat the broth to a simmer in a small saucepan. Add vinegar to the broth and pour over potatoes; stir. Allow potatoes to cool, stirring occasionally. Once cool, pour off any remaining broth that hasn't been absorbed by the potatoes. Season to taste with salt.
Combine the mustard, parsley, chives and oil and stir or shake vigorously (in a small jar) to make a smooth mixture. Pour the mixture over the potatoes and stir gently to coat well. Serve warm or at room temperature.