How to Make It
Melt the butter in a large heavy skillet. Add the sliced onions and cook over moderate heat until slightly softened, about 10 minutes. Reduce the heat to low, add the pearl onions and cook, stirring occasionally, until all of the onions are very soft, about 45 minutes. Season with salt and pepper.
Meanwhile preheat the oven to 350°. Rub the pork loin all over with 1 1/2 teaspoons of salt and 3/4 teaspoon of pepper. Roast the pork for about 1 hour, or until the outside of the meat is browned and the internal temperature registers 150° on an instant-read thermometer. Transfer the roast to a warm platter and discard the strings. Cover loosely with foil and let stand for 15 minutes.
Skim any fat from the juices in the roasting pan, add the sherry and bring to a boil over moderately high heat, scraping up any browned bits from the bottom of the pan. Add the onions and parsley and season with salt and pepper. Thinly slice the pork and serve with the onions and the sherry sauce.