1. Preheat the oven to 400 degrees F. Bring a large pot of water to a boil and cook the barley according to package directions. Cool slightly.
2. On a baking sheet, combine the panko and sage and 1/2 teaspoon of the salt; mix well. Using a pastry brush, coat the pork with the mustard. Roll in the panko mixture. Coat a roasting pan with cooking spray and place pork in pan. Roast 25 minutes.
3. Add the vinegar and oil and remaining salt to the barley; toss. Fold in the walnuts, arugula and apple.
4. Remove pork from oven and let it rest 3 minutes. Slice into 1/2-inch medallions; serve with barley salad.