Meanwhile, wash the parsley, pat it dry, strip the leaves, and finely chop them together with the anchovies, garlic, and pickled gherkin. Transfer the mixture to a mixing bowl.
Hard-boil the eggs, peel them, and separate the yolks from the whites.
Peel the potato while still warm, mash it thoroughly with the tines of a fork, and blend it together with the hard-boiled egg yolks.
Add the potato and yolk mixture to the bowl containing the chopped herbs.
Begin pouring the oil in a thin, steady stream as you would for traditional mayonnaise, whisking continuously with a small whisk.
Season with salt and pepper to taste, then add two tablespoons of white vinegar.
