Score fat cap, but do not removed. Coat Boston butt with mustard outside and inside cavity, followed by rub mixture. Set aside.
Add onions, carrots, celery and garlic to a cast iron Dutch oven.
Place seasoned Boston Butt on top of vegetables.
Pour apple cider and apple cider vinegar over pork roast.
Cover and cook on 250 degrees, baste occasionally, for 5 hours or until internal temperature of the meat reaches 190 degrees.
Take roast from Dutch oven, remove fat cap.
Shred roast with two forks and place in a bowl. Add enough pan juices to the pork to moisten. Strain onions from roasting pan and add to meat.
Taste for seasonings and serve as a main course or sandwiches.