Scrub the potatoes, pick and finely chop the mint, reserving the stalks, then peel and thinly slice the onion. Slice the cornichons into strips.
Cook the Jersey Royals and mint stalks in a saucepan of salted boiling water for about 10 minutes, or until the potatoes are tender. Drain and leave to cool in the colander (for the best flavour, avoid cooling the potatoes by rinsing in cold water).
While potatoes are cooling, place the onion in a bowl, pour over the red wine vinegar and leave to marinate.
Once potatoes are cool enough to handle, crush them slightly with your hands into a large salad bowl, drain and add the onions along with the cornichons and mint. Season, add a splash of oil and serve.
