Scrub potatoes. In a large pot of boiling water, cook potatoes until fork-tender; drain. Shake pan over medium heat for a minute to dry potatoes. Cut into 1/4-inch (5 mm) thick slices. In salad bowl, combine potatoes, parsley and onion.
Tarragon vinaigrette: In a small bowl whisk together vinegar, oil, mutsrad and tarragon; mix well. Pour over warm potatoes and toss to mix. Season with pepper to taste. Cover and let stand at room temperature for at least 1 hour or refrigerate up to 3 days.
Makes 8 servings.