A thick omelette of potatoes and, usually, onion, slowly poached in olive oil, then set with eggs and turned in the pan.
What makes it tortilla de patatas
Potatoes cooked gently in plenty of olive oil, not fried crisp.
Mixed with beaten eggs and left to sit before it goes back in the pan.
Turned over on a plate so both sides set.
Where it comes from
One popular story credits it to the Carlist wars of the nineteenth century, as a cheap, filling food for soldiers; like most such stories, it's hard to prove. What's certain is that it became the most everyday dish in Spain — on every bar counter, cut into wedges or squares, at room temperature.
Whether it has onion is a national argument. So is how runny the middle should be.
It belongs to no single region: it sits here with Madrid's bar food, but it's made and eaten all over Spain.
How it varies
Con cebolla
With onion cooked alongside the potatoes — the more common way.