Preheat the oven to 325 degrees F (165 degrees C). Coat a roasting pan with cooking spray.

Combine stuffing mix and water in a bowl; stir in apple, cranberries, pecans, and onion. Set aside.

Place pork loin on a flat work surface; trim fat and silverskin. Slice pork loin horizontally through the middle to within 1/2-inch of the other side, being careful not to cut all the way through. Open the 2 sides; spread them out like an open book. Place pork loin between two sheets of heavy plastic (resealable plastic freezer bags work well). Firmly pound pork loin with the smooth side of a meat mallet to a thickness of 1/2-inch.

Season pork loin with salt and black pepper; spread reserved stuffing mixture onto cut side of pork loin. Roll pork loin into a firm cylinder; secure with cooking twine. Place in the prepared roasting pan.

Bake in the preheated oven until 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 pork loin with two layers of aluminum foil; rest in a warm area for 10 minutes. Remove kitchen twine before slicing and serving.