Wash the potatoes thoroughly. Steam them for about 20 minutes, or until tender when pierced with a knife. Let them cool slightly.
Meanwhile, peel and finely chop the shallot. Rinse and chop the chives. Slice the cornichons into small pieces.
Prepare the dressing: whisk together the olive oil, vinegar and mustard. Season with salt and pepper to taste.
Cut the potatoes in half lengthwise and arrange them on a serving platter. Sprinkle over the shallot, chives, and cornichons, then drizzle with the mustard vinaigrette. Toss gently to coat and serve warm or at room temperature.
