• Mash together the onion, garlic, oil, ¼ cup port, sage, and pepper. Place the roast in a large shallow container, pouring the mixture over pork, turning to coat. Cover and refrigerate for 24 hours, turning occasionally.
• Remove the roast from the refrigerator and bring to room temperature.
• Preheat oven to 325°F. Prepare a heavy roasting pan with lid.
• Melt the butter in a large skillet and brown the pork on all sides. Place the roast in the bottom of the roasting pan with the marinated juices and spread the vegetables around the base. Pour ½ cup of the chicken stock over the pork and surround with onions. Cover with the lid and gently shake so all the vegetables are covered.
• Bake for one hour, then add another ½ cup of chicken stock to the pan, covering again and give the pan another shake. Cook for another hour, or until a meat thermometer reads 160–165°F or until the juices run clear.
• Remove the roast and tent with tinfoil to rest for 15 minutes or longer which will help retain the juices. Remove the vegetables and tent in a heated serving dish so they stay warm.

Make the Sauce

• Place the roasting pan over medium heat and sprinkle with flour, stirring for one minute. Pour in the remaining stock and ½ cup of port. Bring to a boil and cook until thickened. Strain and season with salt and pepper to taste.
• Slice the roast and plate with the sauce and roasted vegetables.