Melt butter in a skillet set over medium heat. Add onion and apples and cook until tender. Place in bowl. Cool to room temperature and then mix in remaining ingredients, except pork. With a sharp, long and narrow knife make a pocket about 2 inches wide in the centre of the pork loin from one end to the other. Stuff apple/cranberry mixture inside. Place pork in a roasting pan. Season with salt and pepper and roast in a 325 degree F oven about 70-80 minutes, or until internal temperature when checked with a meat thermometer reaches 165 degrees F. Remove roast from pan and allow to rest loosely covered with foil 10 minutes before thinly slicing. Make gravy with pan drippings if desired.