1. Toss onions, cabbage, whole garlic cloves, and 2 tablespoons oil together in a large heavy roasting pan; season with salt.
2. Tear bread and pulse in a food processor or blender with chopped garlic and sage until coarsely ground. Season with salt and pepper.
3. Preheat oven to 450 degree F. Rinse and pat pork dry; season with salt and pepper.Heat a large skillet with remaining 1 tablespoon oil over moderately high heat and brown pork on all sides, about 5 minutes. Place pork, fat side up, in center of roasting pan over onion mixture; spread mustard over top of pork. Sprinkle bread crumb mixture over mustard, pressing to adhere.
4. Roast pork 20 minutes. Reduce oven temperature to 400 degree F; add apples to pan and roast 1 to 1-1/4 hours more, until an instant-read thermometer inserted in center of pork registers 145 degree F, tenting with foil if browning too quickly.
5. Transfer pork in pan to rack; let stand 15 minutes before carving into 1/4-inch-thick slices.
6. Meanwhile, transfer onion mixture to a bowl and stir in vinegar; salt and pepper to taste.
7. Serve pork slices over onion mixture and garnish with fresh sage. Makes 6 servings.