We clean the potatoes, peel and chop them into thin layers. Laminate the onion (if you want onion. You can do it without it).
Fry everything in a pan with olive oil. This is best done on low heat.
Beat eggs and add salt.
When browned, remove the potatoes from heat and drain on paper towels. Pour the potato in the beaten egg and make sure that the eggs cover the potatoes.
We pour a little oil in the pan and when it´s hot we add the eggs and potatoes. We leave it to thicken slightly, about a minute, and with the help of a lid or a plate that has the same radius as the pan, we turn the tortilla. (It is not easy unless you have 2 pans and connect them to do it easier)
We wait another minute and remove it from heat. The egg inside should be done, yet not too dry, as the Tortilla is much tastier when it´s slightly juicy.