Pour a generous amount of olive oil into the pan.
Chop three potatoes and fry them in a generous pool of oil for about ten minutes. Add one chopped onion and fry for about another ten minutes.
Six eggs should do it (though I’ve seen recipes for as low as four to as high as eight eggs).
whisk, whisk, whisk
Drain excess oil from potatoes & onions. Pour beaten eggs over potatoes & onions and let cook for 10-20 minutes. Flip onto a plate and then slide back into the pan to cook for a few more minutes. A cast-iron skillet may not be the best way to go, as they get hot fast and hold heat — I actually burned this tortilla and the egg was overcooked.
For more details, see my previous entry on “la tortilla española”.