Preheat oven to 400 degrees F. Brown sausage in a saute pan, add apple and shallots and cook 3 minutes. Transfer to a mixing bowl, add walnuts and parsley, and season to taste with salt and pepper. Horizontally split pork roast, forming a pocket; do not cut all the way through. Season roast inside and out with salt and pepper. Pack stuffing in center and tie up roast with enough butcher's twine to hold it together. Heat oil in a large saute pan, add roast and sear on all sides until well-browned. Transfer to a rack set in a roasting pan and roast until a thermometer inserted in center registers 170 degrees F. degrees. Remove roast to a carving board, cover loosely with foil and let rest 10 minutes before slicing. Slice roast in 12 pieces. Serve 2 slices per serving, garnished with parsley.