Special equipment: Butcher's string
For the pork: Preheat the oven to 325 degrees F.
Sprinkle the shoulder liberally on all sides with salt and pepper. Truss the roast by taking butcher's string and tying one string tautly lengthwise and 4 shorter strings widthwise (about 1 1/2 inches apart).
Place the roast fat-side up. With a paring knife, make several small incisions into the fat. Cut the rosemary sprigs into 1-inch segments. Place the sliced garlic and rosemary into each incision.
Scatter the chopped onion, carrots and celery in the bottom of a heavy-bottomed roasting pan. Then set the shoulder on top of the vegetables and add 1 quart of water. Place into the oven and roast until the pork registers an internal temperature of 155 degrees F on a meat thermometer, about 2 1/2 hours. Let the meat rest for 10 to 15 minutes before you serve it. Make a pan gravy, or reserve the pan juices and vegetables for another use.
For the pan gravy: Take the roast out of the pan and whisk the flour into the pan drippings. Add 2 cups chicken stock, stirring often over medium heat, until the flour is incorporated. Simmer 20 minutes, and then add salt and pepper to taste. Strain through a fine sieve.
Recipe courtesy of John Besh