Bring a large saucepan of water to the boil and cook the potatoes for 15 to 20 minutes, or until tender but still firm, then drain. Peel while still warm and place into the fridge.
The next day thinly slice potatoes and place into a bowl with gherkins and onion. Season with salt and pepper to taste.
For the mayonnaise:
Combine the egg, egg yolk, mustard and salt in a food processor. Turn on and pour in olive oil very slowly, until it emulsifies with the egg. If you beat the mayonnaise by hand, add oil in drips first and when it emulsifies add it by the teaspoon.
Thin the mayonnaise with a little bit of vinegar, mix with the potatoes and serve immediately or place into the fridge, until time to serve.
Jersey Royals or other new potatoes would be ideal for this recipe.