Heat oil in a 12-inch nonstick skillet over moderate heat until hot but not smoking and add potatoes, onion, and half of salt. Cook over moderately low heat, stirring occasionally, until vegetables are very tender but not colored, about 45 minutes. Drain vegetables in a large colander set over a bowl and cool 5 minutes. Lightly beat eggs in a large bowl. Gently stir in vegetables with 1 tablespoon oil, salt, and pepper to taste. Return 1 tablespoon oil to skillet and add mixture, pressing potatoes flush with eggs. Cook over low heat, covered, 12 to 15 minutes, or until almost set. Turn off heat and let stand, covered, 15 minutes. Shake skillet gently to make sure tortilla is set on bottom and not sticking to skillet. Invert tortilla onto a large flat plate and slide back into skillet, bottom side up. (Alternatively, especially if top is still loose at this point, slide tortilla onto plate first. Cover it with skillet and invert tortilla back into skillet.) Round edge with a rubber spatula and cook over low heat, covered, 15 minutes more, or until set. Slide tortilla onto a serving plate, slice and serve warm or at room temperature.