In a pan place the prunes, apples and wine or stock. Bring to the boil and simmer gently for 15 minutes so that the mixture becomes a rich apple sauce.
Cut the onions in half and cook them in oil for 8 minutes on the flat side, then turn and cook for 8 minutes on the other side, trying to keep the halves intact. Meanwhile, heat the grill. Season the chops and cook on each side for 10-12 minutes. Add the sage and Armagnac to the apple sauce. Take the chops from the grill and serve with the onions and sauce.