Boil the potatoes whole in lightly salted water until just fork-tender.
While the potatoes are still hot, peel them with a paring knife the moment you can handle them.
Slice the potatoes as thinly as you can while still keeping their shape. Transfer to a large serving bowl.
Whisk together the oil, vinegar, cornichon juice, and Maggi in a small bowl. Pour themixture over the potatoes and stir gently to coat. Gently stir in the mayonnaise and then the onion and cornichons. Season to taste with salt and pepper. Cover with plastic wrap and refrigerate for a few hours, but preferably overnight to allow the flavors to meld.