Dice potatoes into small cubes and boil to desired softness. Meanwhile, chop 1/2 onion into rings. Now mix well 4 eggs with 1/2 to 1/4 cup milk. Add 1 tablespoon salt, sprinkle a few dashes of pepper, plus a small liberal amount (about 2 tablespoons) of Spanish or Italian olive oil. Even some diced green peppers are added in the northern provinces of Spain. To this mixture add the onions and potatoes. Pour into large omelette skillet coated with olive oil and simmer at relatively low heat until bottom is well solidified, avoid scraping and mixing while cooking. Then carefully flip it over and cook until there is no more fluid and the tortilla turns golden brown. Serves 4 to 5.