1Place the chopped onions in the bottom of the slow cooker. Place the pork butt on top of the onions and add the garlic, ground mustard, cayenne pepper, salt, pepper, apple cider vinegar and Worcestershire sauce. Pour the Dr. Pepper on top and cook on high for 4-5 hours (or on low for 8 hours).
Very carefully, because the pork will be hot, remove the meat from the slow cooker and place on a large cutting board. Using two forks, shred the pork by pulling away from each other. The meat should be very tender by this point. Place the shredded pork back into the slow cooker and continue to cook for an additional hour.
Drain the remaining juices and toss the meat and onion mixture in the barbecue sauce of choice. I don't have an exact amount listed-just add a bit at a time until you get to your desired sauciness!