Brown sausage in a saute pan. Add apples and shallots and cook 3 minutes. Transfer to a mixing bowl, add walnuts and parsley, and season, to taste, with salt and pepper. Let cool before stuffing.
Split the pork roast horizontally, forming a pocket; do not cut all the way through. Season roast inside and out with salt and pepper. Pack stuffing in the center and tie 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 the center registers 145 to 150 degrees F, about 1 hour to 1 hour and 15 minutes.
Remove roast to a carving board, cover loosely with foil, and let rest 10 minutes before slicing. Slice roast into 12 pieces. Serve 2 slices per serving, garnished with parsley.