Tortilla  serve 4 - 6

    4 large potatoes sliced 3mm thick
    1 large onion thinly sliced  (OPTIONAL)
    4 large eggs
    1 cup olive oil
    coarse salt

Heat oil in a frying pan and add potato slices one at a time to prevent sticking.  
Alternate potato and onion layers salting the layers lightly
Cook slowly over a medium flame.  
Lift and turn potatoes occasionally until tender but NOT brown.

Beat eggs until foamy and salt.
Drain potatoes. ( I put them on kitchen paper to get rid of excess oil)

Add potatoes to the eggs and let to stand for 15 mins
Heat a little oil until VERY hot. 
 
Add the mixture rapidly spreading it out.  

Lower heat to medium high and SHAKE CONTINUOUSLY to prevent sticking.

Turn over. - the tortilla that is.

Cook untill the inside is still a little runny but the outside is cooked.