Tie the roast 3-4 times crosswise, and once lengthwise.
In a bowl, combine garlic, pepper, salt, rosemary, sage and fennel seeds. Rub this mixture well into all surfaces of roast.
Place meat in a roasting pan. Place pan, uncovered, in preheated 300°F oven for about 3 hours.
Add onion to pan around the meat. If there is not much fat in the pan, add the optional oil. Return pan to oven for 3½-4 hours more.
Transfer meat to a dish. Transfer onions to a bowl. Pour drippings into a 2-cup measure. Add water to bring up to 1½ cups. When meat has cooled to room temperature, wrap meat and onions with plastic wrap and place in refrigerator overnight along with drippings.
Remove fat from cold drippings and place in a saucepan with onions, cider, apple jelly and cider vinegar. Bring to a simmer.
Slice cold roast, and layer in baking dish. Pour ½ cup of drippings over pork. Cover dish with foil, and place in a preheated 350°F for 40 minutes until hot.
Bring drippings up to a boil and reduce for 15 minutes until dark, thick and syrupy.
Serve with mashed potatoes on the side. Pour sauce over the meat.