Preparation Heat 1/4 cup oil in a 10-inch nonstick skillet over moderate heat until hot but not smoking, then cook onion with salt and pepper, stirring occasionally, until softened, 3 to 4 minutes. Remove from heat. Peel potatoes and coarsely grate in food processor. Squeeze potatoes in a kitchen towel (not terry cloth) to remove excess liquid, then transfer to a bowl. Add onion mixture (reserve skillet), stirring to coat potatoes, and stir in cheese. Heat 2 tablespoons oil in reserved skillet (not cleaned) over moderate heat until hot but not smoking. Add potato mixture, then press firmly into a cake and round edges with a heatproof spatula. Cover skillet and cook potato cake until underside is crusty, 10 to 12 minutes. Slide out onto a plate. Invert skillet over potato cake and, using pot holders to hold plate and skillet firmly together, flip cake into skillet. Drizzle remaining tablespoon oil around edge of cake, then cover and cook until potatoes are tender and underside is crusty, about 10 minutes. Slide cake onto a platter and cut into wedges.
Preparation Heat 1/4 cup oil in a 10-inch nonstick skillet over moderate heat until hot but not smoking, then cook onion with salt and pepper, stirring occasionally, until softened, 3 to 4 minutes. Remove from heat. Peel potatoes and coarsely grate in food processor. Squeeze potatoes in a kitchen towel (not terry cloth) to remove excess liquid, then transfer to a bowl. Add onion mixture (reserve skillet), stirring to coat potatoes, and stir in cheese. Heat 2 tablespoons oil in reserved skillet (not cleaned) over moderate heat until hot but not smoking. Add potato mixture, then press firmly into a cake and round edges with a heatproof spatula. Cover skillet and cook potato cake until underside is crusty, 10 to 12 minutes. Slide out onto a plate. Invert skillet over potato cake and, using pot holders to hold plate and skillet firmly together, flip cake into skillet. Drizzle remaining tablespoon oil around edge of cake, then cover and cook until potatoes are tender and underside is crusty, about 10 minutes. Slide cake onto a platter and cut into wedges.