Preheat the oven to 400°F. To make the stuffing, melt the butter in a large saucepan on medium. Add celery, onions, and apples and cook until translucent, stirring constantly. Add the sausage, spices, and salt. Cook until brown. Remove from heat and add the cranberries. Set aside to cool.

Either butterfly the pork loin or make a hole in the center of the loin with a sharp knife and work it back and forth to create a pocket the entire length of the pork loin. Place the stuffing inside the loin and secure the entire roll with twine. Place on a cooking rack and cover with foil. Cook for 20 minutes. Remove foil and cook an additional 20-30 minutes or until the internal temperature reaches 165°F. (Cooking times may vary based on size of meat and stuffing distribution.)

Remove from the oven and let cool for 10 minutes before slicing. Remove the twine and slice the pork loin into rounds. Top with feta, if desired, and serve.