Preheat oven to 350 degrees F. Brush roast with mustard and then sprinkle with chopped sage, salt and pepper. Place in a large roasting pan and roast 45 minutes. Toss apples and onions with a little olive oil, salt and pepper and add to pan. Roast 45-60 minutes or until pork reaches an internal temperature of 160 degrees F. Transfer pork to a plate and loosely cover with foil.
Rest 10 minutes to allow juices to set before slicing thinly. Keep apples and onions warm in baking dish. If desired, make gravy with pan drippings or simply serve apple sauce with the slices of pork and apple/onion mixture.