For stuffing, heat butter in a heavy medium skillet over medium-low heat; add ham or sausage and cook 3 minutes. Add onion, celery and a pinch each of salt and pepper; cover and cook, stirring occasionally, until the vegetables are quite soft, about 10 minutes. Stir in diced apple and cook 1 minute. Transfer apple mixture to a large bowl. Sprinkle with sage and stir in cornbread crumbs, apple juice and egg. The mixture should be moist but not wet. Add more apple juice if the mixture seems dry. Taste for seasoning and set aside.
Preheat oven to 450 degrees F. To double butterfly the pork loin, lay the meat, fat-side down, on a work surface and make a horizontal lengthwise cut two-thirds of the way into the depth of the loin and about 1 inch from the long edge nearest you, taking care not to cut all the way through. Flip the loin over so that the cut you just made is opposite you. Make another lengthwise cut, again 1 inch from the edge. Open up the two cuts so you have a large rectangle of meat whose diameter is roughly 3 times the thickness of the meat. Place fat-side down and cover with a sheet of plastic wrap. Using the flat side of a cleaver or a meat pounder, gently flatten the meat to an even thickness.
Remove plastic wrap and spread apple stuffing evenly over the meat, leaving a generous 3/4—inch border. Roll up meat jelly-roll style so that stuffing is in a spiral pattern. Tie rolled roast at 2-inch intervals with butcher’s twine.
Combine 2 teaspoons salt, 1 teaspoon pepper, and remaining sage and sprinkle over roast. Lay the sliced apples on the bottom of a roasting pan just a bit larger than the roast and set the roast, fat-side up, on the apples. Put roast in the oven and cook for 15 minutes, turn the oven down to 325 degrees and roast for 45 minutes. Check the internal temperature of the roast with an instant-read thermometer: The roast is done when it reaches 140 degrees to 145 degrees. If it is not ready, continue to roast, checking the temperature every 10 minutes. When the roast is done, transfer it to a cutting board, tent loosely with foil, and let rest for at least 10 minutes while you make the sauce.
With a slotted spoon, transfer the apples in the roasting pan to a bowl and keep warm. Pour off any fat from the roasting pan, leaving the meat juices on the bottom. Put pan over medium-high heat, add Calvados and deglaze pan allowing the alcohol to burn off, about 15 seconds. Transfer to a small saucepan and add stock, apple juice and cream. Increase heat to high and bring liquid to a boil. Simmer, stirring, until reduced by half. Taste for salt and pepper. Keep the sauce warm while you carve the pork roast.
Remove twine from the roast and cut it into 1/2-inch thick slices. (If you cut the slices too thin, they will fall apart.) Arrange the pork on a serving platter. Spoon the sliced apples around the meat and pour the sauce over all.