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Cut the onions and potatoes with the dark green cone. Season.
Heat oil in the frying pan and add the potato and onion mixture. Cover and cook both sides over medium heat for 15 minutes. Leave to cool in a bowl for 5 minutes. Beat 4 eggs lightly. Pour into the bowl and mix well with the potatoes and onions. Cook the mixture again in the frying pan with a lid.
Cook the tortilla on one side over low heat for 5 minutes and then turn it over and cook for another 2 minutes. Serve in a dish and enjoy the omelette hot.
Variant: Create a hot sandwich by wrapping the omelette in a large slice of Serrano ham.