Heat the oil in a large nonstick frying pan. Saute the onions over medium heat for 6 to 8 minutes or until soft but not brown. Drain the onions in a colander, reserving 2 tablespoons of the oil from the pan. Cook the potatoes in boiling water for 4 to 5 minutes and drain in the colander. Beat the eggs in a large bowl and stir in the onions and potatoes; add salt and pepper to taste. Heat the reserved oil over high heat in a heavy nonstick pan or a well- seasoned cast iron skillet. Add the egg mixture and cook for 2 minutes or until the bottom is lightly browned. Place the pan in a preheated 400-degree oven for 20 to 30 minutes, or until the egg mixture is set. Invert the tortilla onto a round platter. Cut into wedge-shaped slices and serve with a salad. NOTE: In Spain, a tortilla isnt the cornmeal flat bread we think of, but a sort of omelet loaded with onions and peppers. This is a popular item at tapas bars, and an easy dinner to make after an evening of celebrating. Nutrients per Serving: 494 Calories, 15 g Protein, 59 g Carbohydrate, 23 g Fat, 5 g Saturated Fat, 411 mg Cholesterol, 116 mg Sodium. [THE WASHINGTON POST; January 2, 1991]