04While the potatoes are grilling, whisk together the vinegar, mustards, and 1/4 cup oil in a large bowl, add the red onion, cornichon and capers and stir to combine.05Remove the potatoes from the grill and immediately add to the bowl with the other ingredients and gently stir to combine. Season with salt and pepper. Let sit at room temperature at least 15 minutes before serving. Garnish with chopped chervil.