In a medium saucepan, cover the potatoes with water and add 2 tablespoons salt. Bring to a boil then reduce heat and let simmer until tender.
Drain potatoes and slice in half once cool.
Heat the grill to medium.
In a large bowl mix the potatoes with 1/4 cup oil and season with salt and pepper.
Place the potatoes on the grill, cut-side down and grill until lightly golden brown.
Turn the potatoes over and continue grilling until just cooked through.
While the potatoes are grilling, whisk together the vinegar, both mustards, and 1/4 cup oil in a large bowl.
Stir in the red onion, cornichon, and capers.
Remove the potatoes from the grill and immediately add them to the bowl with the other ingredients. Stir gently to combine and season with salt and pepper.
Let the potatoes sit at room temperature for at least 15 minutes before serving. Top with chopped chervil.