1.Place potatoes in a large saucepan of salted cold water. Bring to the boil, then reduce heat and simmer about 15 minutes or until the potatoes are just tender. Drain well, then transfer to a large bowl.
2.Meanwhile, to make mayonnaise, place the egg yolk and juice in a medium bowl; whisk until pale and creamy. Slowly add the oil, whisking constantly, until mayonnaise is thick and creamy. Stir in mustard; season to taste with salt.
3.Sprinkle vinegar over warm potatoes, toss gently. Season with salt and freshly ground black pepper.
4.Add half the mayonnaise, half the herbs and half the onions; toss gently to combine. Drizzle remaining mayonnaise over potatoes, top with remaining herbs and onions.
Not suitable to freeze. Suitable to microwave. For a more substantial potato salad, add some hardboiled eggs or some finely chopped bacon; pan-fry until crisp, then drain on absorbent paper. Toss through salad just before serving.