Tortilla de patatas
Peel the potatoes and either cut them in dice or, for a layered effect, into very 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 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 15 minutes) place a plate over the frying pan and drain off the oil into a heat-proof container. Place the potatoes in a bowl. Beat together the eggs and salt and baking powder, if using, 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. 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 on the plate. Return the oil in the pan, adding a little more if necessary and slide the tortilla back in to cook on the reverse side. Remove the tortilla when it is golden by sliding out on to serving plate. Serves two as a main dish or four to six as an appetizer of first course. Serve with a garnish of salad or vegetables such as pisto, fried chorizo or ham and a tomato sauce, or ketchup.