Preheat the oven to 425 degrees. Place the pork loin on a rack over a roasting pan and fill the bottom with about 1-inch of beer. Season the pork generously with salt and pepper.
In a mixing bowl, combine the orange juice, honey, soy sauce, sherry, hoisin sauce, mashed black beans, fivespice powder, and garlic. Mix together well and coat the pork loin with this paste, rubbing it in well and using up all the mixture. Roast the pork for 15 minutes, then reduce the heat to 325 degrees and cook for 45 to 60 minutes more, basting every 15 to 20 minutes with the orange honey mixture. When the pork is thoroughly cooked (it should register 165 degrees on a meat thermometer, and the juices run clear rather than pink), remove from the oven and rest for 5 minutes. Transfer to a warmed serving platter and surround with orange segments and watercress sprigs. Serve immediately, slicing the loin at the table.
c.1997, M.S. Milliken & S. Feniger, all rights reserved