Decide first what size tortilla you would like (the measurements above are guidelines) and get a good frying pan to match.
Peal and chop into thumbnail sized cubes enough potatoes to fill the pan about three quarters full - the quantity here really depends on your requirements.
Pour olive oil into the pan so that it is about 5mm deep and heat on a medium heat. Add the potatoes to the hot oil. Cook the potatoes slowly as you don't want them to crisp. Move them with a metal spoon regularly until they are soft but not mushy.
While the potatoes are cooking, finely chop an onion. Cut a good section of chorizo - about 2 inches - and peel the skin from it. Slice into 5mm discs and then quarter these. Finely chop garlic to taste.
Once the potatoes are soft, remove from the pan with a slotted spoon to a few sheets of kitchen paper. Drain out most of the oil leaving a good tablespoonful.
Reheat the oil and add the onion, garlic and chorizo and cook until onions are soft and chorizo is almost crispy. Add the potatoes to the pan, season to taste and mix.
Beat enough eggs for the pan size. If it is a large pan you may need about 4 to 5 eggs depending on their size. Pour the eggs over the mixture and give the pan a little shake to make sure the eggs are all through the mixture and then push the mixture gently into the egg so that it is flat on the top. You will know if you have to add another egg at this point.
Gently cook for about 5 to 8 minutes until the edges look cooked and it is starting to firm up. Cook the top of the tortilla under a hot grill until it is golden and cooked. Tip upside down onto a plate as soon as possible by holding a plate over then pan and turning over.
Can be eaten warm or cold. Delicious cold with mayo.
There are lots of different things you can add: mushrooms, peas, courgette, chilli, peppers...