Peel the potatoes, then slice them into thin pieces. Slice the onion as well.
Heat a few centimeters of olive oil in a big pan. Once the oil is very hot, throw the sliced potatoes into the oil and let them cook until they are soft, and some of them start to brown. Don’t forget to stir them occasionally to make sure they don't stick to the bottom of the pan.
You can cook the onion at the same time, or if you prefer, you can cook it after the potatoes are done. The onion should be cooked until it has a soft texture.
Whisk about 5 or 6 eggs in a bowl, depending on their size and the amount of potatoes you have.
Add the potatoes and onion to the bowl, draining the oil from them as much as possible before adding them. Mix everything together with a fork or spoon, and add a pinch of salt. The mixture should be just right; not too eggy and not too thick or dry.
Heat about a teaspoon of olive oil in a non-stick pan. The pan doesn't need to be very big; about 20 or 25 cm wide. Once the oil is hot, pour your egg and potato mixture in the pan. After a few seconds, move the pan a little bit to make sure that the eggs are cooked and the tortilla is not sticking to the bottom. Wait about 20-25 seconds more and then place a plate on top of the pan. It is very important that the plate covers all the pan’s surface, because you need to flip the pan completely so that you get the tortilla in the plate.
Heat another teaspoon of olive oil in the same pan. As soon as it is hot, dump the tortilla back into the pan very carefully. We’re going to cook the other side, so we are going to repeat the same process: cook for 30 seconds, moving the pan slightly and place another clean plate on top of the pan (the one you used earlier has some raw egg that we prefer to avoid just in case it contains nasty Salmonella). Flip it again.
Place your almost done tortilla in the pan once more for about 10 seconds and then it’s ready to serve.