3 medium potatoes, peeled
420 ml (1.66 cups) olive oil
1 large white onion
1. Quarter the potatoes lengthways, then slice them thinly crossways.
2. In a large non-stick frying pan, warm the olive oil over medium heat. When it’s hot, add the potato and cook for about 6 minutes, turning regularly. The slices should cook but not brown.
3. Chop the onion finely and add it to the potato. Mix and cook for another 15 minutes. Strain the mixture using a colander, reserving the cooking oil, and allow the potato and onion to cool.
4. Break the eggs into a large bowl, season with salt and whisk. Add the onion and potato mix to the eggs and stir thoroughly.
5. Pour the reserved cooking oil back into the pan and place it over high heat for 2 minutes. Pour the potato and egg mixture into the oil and shake the pan to spread it evenly. Cook for 4 minutes, then turn the omelette over and cook for another 4 minutes. (Alternatively, you could cook the omelette in an ovenproof frying pan, by putting the pan in a preheated 180C oven for 15 minutes, or under a hot grill/broiler for 5 minutes)
6. Slide the tortilla onto a plate, let it set for 5 minutes, then slice into wedges and serve.