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 edge. Roll up, jelly-roll fashion, starting at one of the short ends. Tie closed with 6 or 7 pieces of cotton butchers' twine. Place onions on bottom of small roasting pan sprayed with cooking spray; top with meat. Mix syrup, mustard and rosemary; spoon over meat.
BAKE 1-1/2 hours or until meat is done (160°F). Let stand 15 min. before slicing to serve.