Heat oven to 350° F. Heat the oil in an ovenproof nonstick skillet over medium heat. Add the potatoes and cook, stirring occasionally, until almost tender, 8 to 10 minutes. Add the onion, 1 teaspoon salt, and 1/4 teaspoon pepper and cook, stirring occasionally, until the onion is soft and the potatoes are tender, about 5 minutes.
Pour the eggs over the potatoes; stir to distribute the ingredients. Transfer the skillet to oven and bake until brown around the edges and a knife inserted into the center comes out clean, 12 to 15 minutes. Serve hot or at room temperature, sliced into wedges. (The tortilla can be made 8 hours in advance and refrigerated; bring to room temperature or reheat before serving.)