Preheat the oven to 350 degrees F (175 degrees C). Place a rack in a shallow roasting pan.

Melt butter in a large skillet over medium heat until foam subsides. Sauté onion in hot butter for about 5 minutes. Stir in apple and celery; sauté without browning, about 5 minutes more. Transfer onion mixture to a large mixing bowl.

Add ground pork, sausage meat, bread crumbs, parsley, salt, pepper, and sage to onion mixture in the bowl; mix gently but thoroughly. (To taste for seasoning, fry a small ball of stuffing in the skillet until pork is cooked through.)

Fill the center of the crown with stuffing, mounding it slightly. Cover stuffing with a round of aluminum foil; wrap the ends of the chop bones in strips of foil to prevent them from charring and snapping off. Place the crown on the rack in the prepared pan.

Roast pork in the center of the preheated oven for about 3 hours, removing foil from stuffing about 30 minutes before pork is done to allow the top to brown. An instant-read thermometer inserted into the center of meat near the bone should read at least 145 degrees F (63 degrees C).

Carefully transfer pork to a large, heated, circular platter. Remove foil from the ends of the chops and replace with paper frills. Let pork rest for about 10 minutes before carving and serving.