Peel the potatoes and either cut them in dice or into thin slices. Heat the oil in the frying pan until very hot and add the potatoes, and onions, if desired. Stir them into the oil to completely coat and seal them, then reduce the heat slightly and continue frying without letting the potatoes brown, stirring them frequently. With the edge of a metal spatula or skimmer, keep cutting into the potatoes, dicing them as they cook. When they are quite tender (about 20 minutes)place a plate over the frying pan and drain off the oil into a heatproof container. Place the potatoes in a bowl. Beat together the eggs and salt until very well combined and stir the eggs into the potatoes and mix well. Return the oil to the frying pan and let it reheat. Now pour in the egg and potato mixture. Let it set on the bottom, regulating the heat so it doesn't brown too fast. (Best in non-stick pan) Use the spatula to firm the edges of the tortilla all around its circumference. Shake the pan frequently to keep it loose on the bottom. Place the plate over the pan, drain off the oil and turn the tortilla out onto the plate. Return the oil to the pan and slide the tortilla back in to cook on the reverse side. Remove the tortilla when it is golden by sliding out onto serving plate

Chopped onion (optional)