Cook the potatoes in salted water until tender but not mushy.
Rinse in cold water - peel them, if you prefer.
Mix the mayonnaise with lemon juice and mustard to your liking, season with salt and pepper.
Version 1: slice the potatoes, eggs and onion into rings, layer them on top of each other in a serving bowl. Drizzle each layer with the mayonnaise mixture. Sprinkle the top with chives.
Version 2: chop the onion, potatoes and eggs roughly into small pieces then in a large bowl toss with the mayo and chives. Season to taste.
Serve cold or at room temperature.