HEAT oven to 350°F.
MELT butter in large skillet on medium heat. Add apples; cook and stir 3 min. or until tender. Remove from heat. Prepare stuffing mix as directed on package. Stir in apple mixture.
PRESS stuffing mixture onto cut-side of meat to within 1/2 inch of edges. Roll up, jelly-roll fashion, starting at one short end. Tie closed with butchers' twine. Spread onions onto bottom of small roasting pan sprayed with cooking spray; top with meat. Mix syrup, mustard and rosemary; spoon over meat.
BAKE 1 hour 5 min. to 1 hour 15 min. or until meat is done (145°F). Remove from oven. Let stand 15 min. before slicing to serve.