Roast pork is one of our favorites, and apples go so well with it. The cider gravy I made up myself and used it on garlic mashed potatoes--this whole meal was served to a group of our local Vietnam Vets and they loved it!
Butterfly the pork loin so that it can be stuffed. Cut kitchen twine into lengths for tying, lay out on work surface, then place roast on top.
Chop onion and celery into the same size chunks. Saute in oil until softened. Stir in apples, also cut to size, then 1 tsp thyme, garlic, and a pinch of regular salt. Saute just 1-2 minutes more, so as not to burn the garlic.
Remove from heat, let cool, and spoon this filling onto the roast, leaving 1 1/2 inches at each end without filling. This will help keep the filling from oozing out during baking. Fold pork over and tie securely.
Sprinkle the outside of roast with coarse salt, ground pepper, and the rest of the thyme.
Place on a rack in a roasting pan, pouring 1 cup chicken broth into the bottom.
Roast at 325 F until meat reaches at least 145 internally. approximately 90 minutes. Let meat rest, loosely tented with foil, while you prepare gravy.
Bring pan drippings plus cider and remaining 1 cup chicken broth to a simmer, scraping up any bits on the bottom of pan.
Combine flour and water til smooth, then slowly add to the gravy, to thicken. use as little thickener as possible, to make a thin gravy.