Step 1Cut the potatoes into quarters, then into thin slices. Heat 1/2 cup of the oil in a very large skillet (about 14 inches) over medium heat. Add the potatoes and cook, covered, turning with a spatula occasionally until they are half-done, stirring occasionally, 10 to 15 minutes. Add the salt and onion and continue cooking, uncovered, until the onion slices are wilted and the potatoes are tender but not collapsing, stirring occasionally, 10 to 15 minutes. Remove the skillet from the heat and transfer the mixture to a large bowl to cool slightly. Some of the vegetables may stick to the pan; do not scrape them off.
Step 2In a second bowl, beat the eggs. Pour over the potatoes and thoroughly fold in. Heat the oven to 375 degrees.
Step 3Heat a 10-inch nonstick oven-safe skillet briefly over high heat. Add the remaining tablespoon of oil and continue to heat, almost to the point of smoking. Carefully swirl the pan to coat the bottom and sides with oil. Pour the potato mixture in, smoothing out the top, and leave over high heat for 1 to 2 minutes to sear the bottom. Lower the heat as much as possible and let cook, uncovered, until the potato mixture has set fairly well but the center is still wet, about 10 to 15 minutes.
Step 4Transfer the skillet to the oven and bake until the center of the tortilla is completely set and feels firm to the touch, 10 to 15 minutes longer. Carefully slide out of the skillet onto a plate, then invert onto a second serving plate so that the crusty bottom is on top. Serve hot or at room temperature, in skinny slices.