For stuffing, stir together the chopped apple, bread crumbs, celery, raisins, walnuts, green onions, and nutmeg. Stir in 1 tablespoon of the apple juice or 1 tablespoon apple cider.
To butterfly the pork tenderloin, make a single lengthwise cut down the center of one narrow edge of the meat, cutting to within 1/2-inch of the other side. Spread the meat open. Place knife in the "v" of the first cut. Cut horizontally to the cut surface and away from the first cut to within 1/2-inch of the other side of the meat. Repeat on the opposite side of the "v". Spread these sections open.
Cover meat with clear plastic wrap. Pound with a meat mallet to 1/2-inch thickness. Spread stuffing over meat. Roll up from one of the short sides. Tie with string to secure. Brush with some of the remaining 1 tablespoon apple juice.
Place meat on a rack in a shallow roasting pan. Roast, uncovered, in a 350 degree F oven for 1 to 1-1/4 hours or meat thermometer registers 170 degrees F, brushing with the remainder of the 1 tablespoon apple juice after 30 minutes.
Meanwhile, in a small saucepan stir together the apple juice or cider, the cornstarch, and cinnamon. Cook and stir until thickened and bubbly. Cook and stir for 2 minutes more. Serve with roast. Makes 4 servings.