Heat the oil in a 25-30 cm non-stick fry pan and add the potatoes, they should all fit in one batch. Cook on a gentle heat until soft not brown. Remove from pan and then add the onions to the hot oil and cook until they are soft and sweet then remove from oil. Drain the hot oil leaving behind around 30ml. In a large bowl beat the 12 eggs, 1 teaspoon salt, then add the warm potato and onion mix well. Return the mixture to the pan. The pan must be hot when the mixture is returned otherwise is may stick to the sides making it difficult to flip. Reduce the heat to medium and cook for 5-8 min. Place a plate on the top of the pan and gently flip the tortilla onto the plate. Slide the tortilla back into the pan and cook for a further 5 minutes. Serve hot as is or with garlic aioli.