In a large skillet, melt the butter in the vegetable oil. Add the sliced onions, cover and cook over moderate heat, stirring, until softened, about 12 minutes. Uncover the skillet and cook, stirring occasionally, until the onions are very soft and well browned, about 45 minutes longer; add a little water as necessary to prevent the onions from sticking. Add the Marsala and cook until nearly evaporated. Season the onions with salt and white pepper and transfer them to a medium bowl.