There are dishes that became famous because they are expensive, and dishes that became famous because everyone made them when they had nothing else. Andalusian papas aliñás are the second kind: boiled potato, onion, hard-boiled egg, parsley, vinegar and olive oil. Nothing more.

In Cádiz this is a tapa eaten at room temperature, in a bar, with a glass of something cold. Which is exactly why it is perfect for August - no oven, no standing over a hob, and it is better after it has sat for a while.

What you need

One kilo of potatoes, small to medium, from varieties that hold their shape after boiling. One shallot or half a sweet onion. Three large eggs. A bunch of fresh parsley. Extra virgin olive oil, sherry vinegar and salt.

The sherry vinegar is not a whim here. In a recipe with six ingredients there is nowhere for a bad one to hide - the oil and the vinegar are half the flavour, so this is not where you economise.

The method

Wash the potatoes well so there is no soil left, and put them into boiling salted water. The eggs go in at the same time as the potatoes. After nine to ten minutes take the eggs out and cool them under running water - the yolk stays creamy, not grey.

The potatoes carry on for another twenty to twenty-five minutes, or up to thirty if they are larger. They are done when a fork goes in without resistance. While they boil, slice the onion into thin strips or dice it and soak it briefly in water - that takes the sharpness out. Chop the parsley finely, leaves only.

Once the potatoes have cooled a little, peel them and tear them into irregular pieces straight into the serving dish. Peel the eggs and break them over the potatoes. Add onion and parsley, salt generously, pour over the vinegar and the olive oil and mix so everything gets coated.

One rule that makes the whole difference

Dress the potatoes while they are still warm. That is the entire secret of the dish and the only place you can get it wrong. Cold potato does not absorb - the oil and vinegar stay on the surface and end up at the bottom of the bowl. Warm potato pulls them inwards, which is why every bite has flavour, not just the first.

Second: if you can, do not put it in the fridge. Chilling changes the structure of the potato and turns it from creamy to dense. If you must, take it out at least half an hour before eating.

In Cádiz they often add chunks of tuna. That is not obligatory, but it is a common home addition and it turns a tapa into lunch. With it or without, the bill for four will not be higher than one coffee in the city centre - which explains rather well why the dish has survived this long.