Peel and slice the potatoes very thinly. Chop the onion and place together with the potatoes in a frying pan. Cover with Spanish olive oil and fry slowly until soft (20-25 minutes), turning them occasionally and gently breaking the potatoes with a fork. Drain the potatoes in a sieve, reserving one tablespoon of the Spanish olive oil in which they cooked. The rest of the Spanish olive oil can be stored and reused next time you make a potato omelet. In a large bowl, beat the eggs with spices. Add the fried potatoes, mix well and season with salt. Leave the entire mixture to rest for a time. Heat the reserved tablespoon of Spanish olive oil in the same pan until it is very hot. Add the egg mixture, lower the heat, and cook for 5 minutes. Use a plate to carefully turn the omelet, and continue to cook for 5 minutes or until golden brown. Spanish tortillas can be served hot or at room temperature.