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 apple cider.
Butterfly the pork tenderloin (see small photo above).
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° oven for 1 to 1 1/4 hours or till meat is no longer pink, brushing with the remainder of the 1 tablespoon apple juice after 30 minutes.
Meanwhile, in a small saucepan stir together the 1/2 cup apple juice or apple cider, cornstarch, and cinnamon.
Cook and stir till thickened and bubbly.
Cook and stir for 2 minutes more.