Fry the potatoes and onions in oil until they begin to turn golden brown, then remove from the pan and drain well to remove excess oil. When the mixture has cooled, add to the beaten eggs and season with salt and pepper. Heat a little olive oil in a frying pan and add the egg mixture, swirling slightly and cook over a low to medium heat until the bottom is quite firm and starts to brown. Turn it over and cook on the other side. To do this, simply place a large plate over the pan, hold the plate firmly in place and turn the whole tortilla over. Quickly slide the tortilla back into the pan and continue cooking. Remove from the pan and serve warm, or at room temperature. The finished tortilla should be quite firm but with a soft interior, and 1 to 2 inches thick. Often served with fried green peppers decorating the top, which if left for several hours, imparts a wonderful flavor to the dish.
Variations: Reduce the amount of potatoes and add peas, peppers, asparagus and other vegetables. You can also add ham and/or chorizo to make a nice variation.