Preheat broiler. Place the brisket on a rack in a broiling pan. Broil until the outside is browned on both sides. Preheat oven to 350 degrees. Heat the beef fat in a large roasting pan or a 7to 8-quart heat-proof casserole. Saute the onions in the fat. When the onions are brown, stir in the stock. Place the meat in the pan or casserole, cutting it in half if necessary. Cover the pan. Place the pan in the oven. Bake for three to four hours, or until very tender. Allow to cool. Refrigerate overnight. Remove congealed fat from the sauce. Reheat the meat in the sauce. Season with salt and pepper. Slice and serve the meat in the sauce or with sauce on the side.