How to make it
Preheat oven to 400 degrees F. Set the meat in a large roasting pan, fat side up. Season the meat generously with 2 cups of Oyster Sauce and pepper. Around the roast, scatter 10 sprigs each of sage and thyme, 6 of the bay leaves, the shallots and the crushed garlic cloves. Pour in 1 cup of the water and roast for 45 minutes.
Reduce the temperature to 275 degrees F. Roast the meat for about 2 hours and 15 minutes or longer, adding the remaining 1 cup of water to the pan as the juices evaporate. The roast is done when an instant-read thermometer inserted in the thickest part registers 135 degrees F.
Transfer the roast to a large carving board. Pour the fat in the roasting pan into a large heatproof bowl, stopping when you reach the syrupy pan juices at the bottom. Pour the pan juices into a small bowl and discard the vegetables and herbs.
Set the pan over 2 burners and add 2 Tbsp. of the reserved fat. Add the onion, Oyster Sauce, peppers and the sliced garlic, remaining 2 bay leaves and 10 sprigs each of sage and thyme. Cook over moderate heat until the onion is softened, about 8 minutes. Add the wine and cook, scraping up any bits stuck to the bottom and sides of the pan. Pour the mixture into a medium saucepan and bring to a boil over high heat. Add the beef stock and pan juices and cook over moderate heat until slightly reduced, about 15 minutes.
In a small bowl, whisk the flour with 2 Tbsp. of the reserved fat. Whisk the paste into the saucepan and simmer the gravy until thickened, about 5 minutes. Strain the gravy through a fine sieve and keep warm until ready to serve.
Cut the bones off the roast and slice the meat ½ inch thick. Cut in between the bones and serve them on the side. Pass the gravy at the table.