All you need are some potatoes, some eggs and a large non-stick pan. However, to make this tortilla tastier, you should add some veg and if are not vegetarian, some kind of meat. For a real taste of Spain, choose some chorizo, but prawns or bacon are also an option.
First, you need to boil some potatoes, about 500g, with their skin on, until they are cooked but not too soft (we are not doing mash). Then, cool them while getting the rest ready.
Slice a couple of big onions and fry them in some extra virgin olive oil along with some chopped veg, peppers or mushrooms. If using chorizo, start frying it without oil and use the natural oil to fry the rest.
Once the potatoes are cold, peel them with a knife and roughly slice them. Add them to the pan and gently mix with the rest.
In a bowl, break between 4 and 6 eggs, whisk and season. Herbs such as parsley, rosemary, tarragon or basil add a great taste to a tortilla. Pour the onion and potato mix into the eggs and mix everything together.
In the pan (preferably clean), heat some olive oil until pipping. Pour the egg mix into the pan and flatten with a wooden spoon or a spatula. After a few seconds, shake the pan from left to right and back a few times to ensure the mix doesn't stick to the bottom.
Reduce the heat and cook for a few minutes. Once the inside looks solid, cover the pan with a plate or a board, and turn the tortilla on it. Return it to the pan so the other side will cook, or put the pan under a grill.
Serve hot or cold on a bed of salad, with some bread and a dash of olive oil. Buen provecho!
For more information about the tortilla, check this.