Preheat an oven to 325 degrees F (165 degrees C). Combine the stuffing mix and water in a mixing bowl; set aside. Grease a roasting pan with cooking spray.
Trim the fat and connective tissue from the pork loin. Cut from one side through the middle horizontally to within 1/2-inch of the other side. Open the two sides and spread them out like an open book. Place the pork loin between two sheets of heavy plastic (resealable freezer bags work well) on a solid, level surface. Firmly pound the pork with the smooth side of a meat mallet to a thickness of 1/2-inch.
Stir the apple, cranberries, pecans, and onion into the stuffing mix. Season the pork loin with salt and pepper to taste, then spread the stuffing mix onto the cut side of the loin. Roll the pork into a firm cylinder, and secure with cooking twine. Place into the prepared roasting pan.
Bake in the preheated oven until the pork is no longer pink in the center, about 1 hour. An instant-read thermometer inserted into the center should read 145 degrees F (63 degrees C). Cover the meat with two layers of aluminum foil, and allow to rest in a warm area for 10 minutes. Remove kitchen twine before slicing and serving.
