Put the roast in a large roasting pan, and roast 15 minutes. Reduce the oven temperature to 400 degrees. Tilt the roasting pan, and spoon off excess fat. Scatter the carrot, celery, onion, and garlic around the roast. Roast an additional 15 minutes. Add the reserved prunes and soaking liquid, and roast 10 minutes. Pour the stock into the pan, and continue cooking, basting the roast occasionally with the pan juices, until a meat thermometer inserted into the thickest part of the roast registers 145 degrees, about 30 minutes more.