I found this recipe in my cookbook called "Four Ingredient Cooking", by Joanna Farrow, it is described as best-known Spanish tortilla. You can add red/yellow peppers, cooked peas, corn, grated Cheddar or Swiss cheese into the mixture in step 2, if you like, and season to taste with salt, pepper and any herbs.
- Thinly slice the potatoes.
- Heat 5 tablespoons of the oil in a skillet and cook the potatoes and onions, turning frequently, for 10 minutes more, until the vegetables are tender.
- Meanwhile, beat the eggs in a large bowl with a little seasoning.
- Tip the potatoes and onions into the eggs and mix gently. Let stand for 10 minutes.
- Wipe out the pan skillet with paper towels and heat the remaining oil in it.
- Pour the egg mixture into the skillet and spread it out in an even layer. Cover and cook over very gentle heat for 20 minutes, until the eggs are just set.
- Serve cut into wedges.