1) Cover potatoes with water in a medium saucepan. Add 2 tablespoons salt. Bring to the boil; reduce heat, and simmer until tender, about 8 minutes. Drain potatoes, let cool and slice in half.
2) Heat the barbecue to medium.
Brush 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 griddle until lightly golden brown, about 4 minutes. Turn the potatoes over and continue until just cooked through, about 4 minutes longer.
3) Whilst potatoes are grilling, whisk together the vinegar, mustard and oil in a large bowl, add the red onion, gherkin and capers. Stir to combine.
4) Remove the potatoes from the griddle. Add immediately 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.