This recipe is for a Spanish omelette called Tortilla Espanola. It contains potatoes, onions, eggs, and olive oil. To make it, thinly slice potatoes and onions and cook them in olive oil until soft. In a separate bowl, whisk eggs until fluffy. Pour the egg mixture over the cooked vegetables and fry until set on both sides. Slice and serve. The omelette can be served as a light meal or sandwich filling.