Serves 8 to 10.
1. In a 10-inch skillet, heat the oil and when it is hot, add the potatoes, onion, and salt. Cook over medium heat for 15 to 20 minutes, stirring occasionally, until the potatoes are tender. Set a colander over a bowl; drain the vegetables. Reserve 2 tablespoons of the oil. Wipe the skillet, scraping off stuck pieces.
2. In a large bowl, beat the eggs with a fork. Stir in the potatoes, onion, and pepper. Press the vegetables with a spatula so they are completely covered by the eggs. Let the mixture sit for 15 minutes.
3. In the skillet, heat the reserved 2 tablespoons oil. When it is hot, add the egg mixture, pressing the potatoes into the eggs. Cover the skillet and cook over low heat for 12 minutes. Turn off the heat and let the skillet stand, covered, for 12 minutes more, shaking the skillet gently to make sure the tortilla is set and not sticking to the pan.
4. Hold a large flat plate over the skillet and turn the skillet upside down so the tortilla slips out. The bottom side will be on top. Now slide the tortilla back into the skillet to cook the other side. Cover and cook over low heat for 12 minutes, or until the tortilla is set.
5. Slide the tortilla onto a serving plate and serve warm or at room temperature, cut into cubes or thin wedges.
ANDREA PYENSON