First of all, peel and cut the onion in half, then thinly slice each half and separate the layers into half-moon shapes.
Now thinly pare the potatoes using a potato peeler and slice them into thin rounds – you have to work pretty quickly here because you don't want the slices to brown.
When they are sliced, rub them in a clean tea cloth to get them as dry as possible.
Now heat 1 tablespoon of the oil in the pan, and when it’s smoking hot add the onions and potatoes.
Toss them around in the oil to get a good coating and add the salt and some freshly milled black pepper and a teaspoon of salt.
Turn the heat down to low, put a lid on the pan and let them cook for 10–12 minutes or until tender.
You can turn them over half way through and shake the pan from time to time. They are sort of stewing in the oil rather than browning.
Meanwhile whisk 4 large eggs very lightly in a jug, then quickly add the cooked potatoes and onions and give them a good mix to coat with the egg.
Then put the pan back on the heat and add half a tablespoon of oil, turning the heat up to medium to heat the oil.
Now pour the whole lot back into the pan. Turn the heat down to low and give the tortilla 20-25 minutes without a lid, and from time to time push the edges in all round with a quite firmly with a palette knife in a vertical position to give it that traditional rounded edge.
When it’s ready there will be virtually no liquid egg left – but you now need to turn it over or if you don’t fancy that it can be finished under the grill but its very easy to turn it.
To do this use a flat lid, place it over the pan and invert to get the tortilla onto the lid. Put the pan back on the heat and use a palette knife to gently ease the tortilla back in.
Give it 2 minutes, then turn the heat out and let it settle for 5 minutes.
Serve a tortilla cut into wedges, warm with a salad. It’s also good served cold for a picnic or cut into cubes to serve as an appetizer.