Cover potatoes with water. Add 2 tbsp. salt. Bring to a boil, reduce heat, simmer until tender, about 8 minutes. Drain, cool, slice in half.
Heat grill.
Toss potatoes with 1/4 cup EVOO and season with salt and pepper.
Whisk together the vinegars, mustards, and 1/4 cup EVOO in a large bowl. Add the red onion, cornichon and capers.
Grill potatoes 4 minutes per side or until done.
Add potatoes straight from the grill to the marinate. Season with salt and pepper. Let sit for 15 minutes, then garnish with the chervil or cilantro.