1.In a large saucepan, cover potatoes with cold water; bring to boil. Reduce heat; simmer, covered, until tender. Drain; cut into 3cm pieces. Spread potato on a tray, sprinkle with vinegar; refrigerate until cold.
3.Combine potato in large bowl with mayonnaise, onion and parsley.
4.Blend or process egg yolks, juice and mustard until smooth. With motor operating, gradually add oil in a thin, steady stream; process until mixture thickens. Add as much of the warm water as required to thin mayonnaise.