Season the shoulder liberally on all sides with salt and pepper. If you wish you can even season it overnight.
Truss the roast by taking butchers string and tying one string tautly lengthwise and 4 shorter strings crosswise (about 1-2cms apart).
Place the roast fat side up. With a paring knife, make several small incisions into the fat. Cut the rosemary sprigs into 2-cm segments and insert into every other cut so that half the sprig is inserted into the fat. Then in the remaining incisions, place the sliced garlic.
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 about two and half hours or until the pork registers an internal temperature of 70°C on a meat thermometer. Let the meat rest for 10-15 minutes before you serve it. Reserve the pan juices and vegetables for another use or make a pan gravy.