The Spanish omelette or Spanish tortilla, also called simply tortilla in English when there is no confusion with the Mexican maize tortilla, is a typically Spanish dish consisting of a thick egg omelette made with potatoes and fried in vegetable oil.
The Spanish tortilla (Tortilla de patatas in Spanish) is the most common (although not the only one thorough) gastronomic specialty you can find all over Spain. There are hundreds of variations even in one specific region but the most common is the one made with eggs, potatoes and onion. There is not a standard tortilla.
The potatoes, ideally starchy rather than waxy ones, are cut into thin slices or in small dices. They are then fried in vegetable oil until they soften. Then you add the sliced onions at a moderate temperature until they are soft, but not brown. Browning is often avoided by an excess of vegetable oil, which can later be strained and reused. The potatoes and onions are then removed, drained, and mixed with raw beaten and salted eggs. This mixture is then returned to the pan and slowly fried. The tortilla is fried first on one side and then flipped over to fry on its other side. Flipping is accomplished with the help of a plate or a "vuelve tortillas" (a ceramic or wooden lid-like utensil made for this particular purpose). The plate or “vuelve tortillas” is placed on top of the pan and then, with one hand on top of the plate and the other holding the pan, both are inverted, leaving the tortilla upside-down on the plate. The tortilla is then slid carefully back into the pan. Other ingredients, like green or red peppers, chorizo, tuna, shrimps or different vegetables, can be added. It can also be made without onions.
The tortilla may be eaten hot or cold; it is commonly served as a snack (tapa) or picnic dish throughout Spain. As a tapa, it may be cut into bite-size pieces and served on cocktail sticks, or cut into pie style (triangle) portions (pincho de tortilla).