Stuffed veal pockets with cider sauce

3 tablespoons butter
1/2 medium onion, sliced
2 Granny Smith apples
2 teaspoons apple cider vinegar
1 pinch salt
3 cups sparkling cider
1-1/2 cups brown gravy
1-1/3 pounds veal scaloppini (about 12, 1-1/2 ounces each)
1/4 teaspoon salt
1/4 teaspoon freshly ground black pepper
Olive oil

1. In a saute pan, melt the butter and saute the onions until soft. Peel, core, and thinly slice the apples. Add to the onions, and saute until the apples are soft, about 3 to 5 minutes more. Remove from the heat and add the vinegar and salt. Refrigerate until cold.
2. In a medium saucepan, simmer the sparkling cider until only 1/2 cup remains, 30 to 45 minutes. Whisk in the brown gravy; simmer gently for 10 minutes. Remove from heat.
3. Lay a few scaloppini on a long sheet of waxed paper. Cover with another sheet of waxed paper. With the flat side of a meat mallet, pound the scaloppini until very thin, then lay them on a clean work surface. Repeat with the rest of the scaloppini, working in batches. Season the flatten scaloppini with salt and pepper. Top each with an equal amount of the chilled apple mixture. Fold each scaloppini in half to make a pocket and close with a toothpick. Brush pockets with oil. Grill over Direct High heat for 4 to 6 minutes, turning once halfway through grilling time.
4. Reheat the sauce and serve with the veal pockets.