Cover potatoes with water in a medium saucepan. Add 2 tablespoons salt. Bring to a boil; reduce heat, and simmer until tender, about 8 minutes. Drain potatoes and let cool and slice in half.
Heat the grill to medium.
In a large bowl toss the potatoes with 1/4 cup of the oil and season with salt and pepper. Place the potatoes on the grill, cut-side down and grill until lightly golden brown, about 4 minutes. Turn the potatoes over and continue grilling until just cooked through, about 4 minutes longer.
While 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.
Remove 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.