In a large cast-iron skillet, heat the remaining 1/2 cup of oil until shimmering. Add the potatoes in an even layer and cook over moderate heat until browned on the bottom, about 15 minutes; lower the heat halfway through cooking. Stir the potatoes and cook until browned all over, about 10 minutes longer. Add the onion and cook over moderate heat, stirring, until beginning to brown. Season with salt and pepper.