Scrub the potatoes clean with paper towel, and transfer to a large pot. Add enough cold water to cover. Cover and bring to a boil over high heat. Reduce heat to medium-low and simmer until potatoes are done. Begin checking after 10 minutes by poking the potato with a paring knife. Potatoes are done when they are tender all the way through. The knife should easily get through, but not break the potatoes. Don’t over cook.
Carefully remove the potatoes into a large colander to cool a bit. Peel potatoes while still warm. While the potatoes are cooling, chop the onions and gherkins.
Cut potatoes in cubes and transfer to a large bowl. Sprinkle with the spices and add the gherkins.
Add the onions and 125 ml cold water to a small pot. Bring to a boil and let cook, uncovered, for 2 minutes. Pour immediately (with the water) over the warm potatoes.
Add the oil and vinegar and toss to combine. Adjust taste if needed. Let stand for 30 minutes before serving. Taste best when lukewarm or at room temperature