Fill a pan with cold water and add a pinch of salt. Add the potatoes and bring to a gentle boil (if the water is bubbling too much it will break up the potatoes). Cook until very soft, then remove with a slotted spoon to a colander to drain – if you pour them out they will break up. While they are still warm, peel the potatoes with a small knife and cut into roughly 3cm chunks.
Fill a smaller pan with water and bring to the boil. When it’s bubbling, add the eggs and boil for 10 minutes, then drain and peel them under running water (it’s easier to peel them this way). Cut them into quarters.
Slice the onion very thinly into julienne. Roughly slice the parsley leaves – don’t chop them, you want to eat them as part of the salad.
Put the potatoes on a platter, and top with layers of egg, onion, parsley and tuna.
Cover and refrigerate for around 20 minutes, or until it’s nice and cold. When you’re ready to eat, drizzle over the olive oil and vinegar and season with salt and pepper. Serve with bread, to scoop up the juices.