Step 1Steam the potatoes: Arrange them in a single layer in a steamer basket and place the basket over a pan of rapidly boiling water. Cover and cook until they are tender, about 15 minutes (the time will vary depending on the size of your potatoes). As soon as they are cool enough to handle without scorching your fingers, peel the potatoes, if you like. Cut the potatoes crosswise into half-inch pieces; if using fingerlings, quarter them lengthwise.
Step 2While the potatoes are steaming, bring the broth to a simmer in a small saucepan. After cutting up the potatoes, combine them with the onion in a mixing bowl. Add the vinegar to the broth and pour the mixture over the potatoes. Set aside until the potatoes have cooled, stirring gently from time to time to coat well. When the potatoes are cool, pour off any broth that remains. Season to taste with salt.
Step 3In a small bowl or lidded jar, combine the mustard, parsley, chives and oil and stir or shake vigorously to make a smooth mixture. Pour the mixture over the potatoes and stir gently to coat well. Serve warm or at room temperature.