For the vinaigrette, whisk together the vinegar, garlic, mustard, salt, and pepper in a large bowl for 30 seconds to dissolve the salt. Gradually whisk in the oil so the dressing emulsifies and thickens slightly. Whisk in the chervil and thyme, taste, and adjust the seasoning. Set aside.
Put the potatoes in a large pan of salted water, cutting any large ones into 2 or 3 uniform pieces s