Scrub the potatoes. Peel and thinly slice the onion. Dice the gherkins.
Finely chop the mint, reserving the stalks.
Put the potatoes and mint stalks in a saucepan, add a little salt and bring to the boil. Simmer for 10 minutes until the potatoes are tender. Drain and let them cool in the colander.
Meanwhile, put the onion slices in a large bowl, pour over the vinegar and leave to marinate.
When the potatoes have cooled, put them into the bowl, together with the gherkins and mint. Season with salt and pepper, then drizzle with olive oil. Toss well.
If you have time, allow the flavors to infuse for 15-30 minutes before serving.