(Universidad Complutense de Madrid)
Fun story behind this recipe:
This is one of the main staples of the Spanish traditional fare, together with the garlic soup and diverse variations on the "cocido" - basically a dish of chickpeas with added vegetables and pieces of meat.
1 pinch of salt
Olive oil (sunflower oil will also do)
Peel the potatoes, wash them, dry them, slice them, and sprinkle with salt. Chop the onion.
Pour some oil in a frying pan, heat it, then stir in the chopped onion. When the onion turns golden, add the sliced potatoes. Fry until the potato gets nice and crispy. Place the fried potato in a dish, then pour a little more oil into the pan and heat again.
Break the eggs and whip them up (if you whip the eggs' whites first for a while, the omelette will taste juicier). Mix the battered eggs with the fried potato. Pour into the pan, making sure batter shapes into a nice circle. Wait until the underside of the omelette is done (make sure it doesn't get stuck to the bottom of the pan by using enough oil and slightly shaking the pan every now and then).
When the underside is cooked (more a less, after 3 minutes or so), you have two alternatives to flip the omelette over. Either you risk tossing it up into the air (really not quite recommendable with this sort of heavy omelettes), or you turn it over with the help of a large dish. Allow the other side to cook until it is done.
You can turn the omelette over until it suits your preferences. The more cooked, the more compact and drier it will be. Less-cooked omelettes tend to be softer and runnier.
You can also add other fried vegetables to the mix, e.g. red or green peppers, aubergine, tomato. Also, you can substitute the onion for chopped garlic (a clove or so).