Melt butter in large nonstick skillet on medium heat. Add apple; cook and stir 3 minutes or until soft. Remove from heat.
Meanwhile, prepare stuffing mix as directed on package. Stir in cooked apple mixture.
Press stuffing mixture onto cut-side of pork loin to within 1/2 inch of edge. Roll up, jelly-roll fashion. Tie closed with 6 or 7 pieces of cotton butchers' twine. Place onion on bottom of small roasting pan sprayed with no stick cooking spray. Place pork on top of onion.
Mix maple syrup and mustard. Stir in rosemary. Spoon mixture evenly over pork.
Bake at 350°F for 1-1/2 hours or until meat thermometer registers 160°F. Remove from oven. Let stand 15 minutes before slicing.
