Preheat oven to 300°. Melt 4 tbsp. of the butter in a large nonstick skillet over low heat. Finely chop 2 of the onion halves and add to the skillet along with the celery, garlic, and sage. Increase heat to medium and cook, stirring often, until soft, 3–5 minutes. Remove skillet from heat, stir in breadcrumbs, and season stuffing to taste with salt and pepper. Use a sharp knife to make a deep pocket in the side of each chop, fill each with a quarter of the stuffing, then close each one with a toothpick. Season chops with salt and pepper.
Wipe skillet clean and return to medium-high heat. Brown stuffed chops, 2 at a time, making sure that both sides and the edges get nicely browned, about 3 minutes per side. Transfer chops to a large roasting pan.
Reduce heat to low and melt remaining 4 tbsp. butter in skillet. Put melted butter, apples, calvados, and remaining onions in large bowl and season to taste with salt and pepper. Toss well. Increase heat to medium-high, put apples and onions in the skillet, and sauté until evenly browned all over, 8–10 minutes. Scatter around the chops in the roasting pan. Cover pan tightly with foil and roast until chops are tender, about 3 hours. Remove toothpicks before serving.