The standard base for a tortilla (omelette) is eggs and potatoes (patatas). But in different regions and in different households there may be some additional ingredients. For example, I like onion in my tortilla. And in Valencia you commonly find pea and potato tortillas.
My friend Victor, whose Dad is Spanish, first showed me how to make a tortilla with a recipe that he brought back from Barcelona that he learned from his uncle.
It is simple enough. Slice waxy potatoes thinly. You will need enough to fill a large frying pan, even though the tortilla is going to be made in a small but deep frying pan. This is when I also fry my onions.
You will need a lot more oil than you think, so, when the potatoes are ready, you need to put them in a colander and drain off the excess oil. This also allows the potatoes to cool, which is important because you don't want hot potatoes cooking your eggs too quickly. You can reuse the excess oil that you drain off.
Once the potatoes are cool and drained, put them in a small deep frying pan. Bear enough eggs to cover the potatoes - this could be as much as half a dozen - and add salt before pouring into the pan with the potatoes.
Now you have 2 options. You can cook the omelette mostly through, then finish up under the grill. Or you can try and flip it by using a plate. This way is a little messier, but I find it is better. The reason it is better is because the best tortillas are still slightly runny on the inside and flipping it achieves this better than the grill approach.
The other thing to note with flipping is that you don't have to do it just once. You can flip every couple of minutes. So, if you make a mess of the top, don't worry coz you're going to flip if again soon. I see this kind of like making a quenelle. Flipping back and forth will eventually achieve nice smooth edges. Unless, of course, the pan you're using is no longer non-stick. Then it would just be an irretrievable hash.
And there you have it. If you want to jazz your tortilla up, try adding green beans, or red pepper and chorizo, or maybe asparagus and little prawns. You get the picture!