Trim the skin off the ham, leaving a 1/4-inch-thick layer of fat. Score the fat in a shallow cross-hatch pattern. Spread the onion slices in a large roasting pan. Set the ham on top and add 1/2 cup of the stock. Cover the ham with parchment paper, then cover the roasting pan with foil. Roast the ham for 1 1/4 hours.