Trim any large, excessive pieces of fat from the pork roast, leaving a small fat cap.
Pat the meat completely dry with paper towels. Generously season all sides with the dry rub, taking a moment to massage the spices into the meat.
Arrange the sliced onions and whole garlic cloves in an even layer on the bottom of a large slow cooker.
Pour in the apple cider vinegar and water.
Gently place the seasoned pork roast on top of the onions and garlic, fat-side up.
Cover and cook on LOW for 8 to 10 hours or on HIGH for 4 to 5 hours. The pork is ready when it is exceptionally tender and easily shreds with a fork.
Carefully remove the pork from the slow cooker and let it rest on a cutting board for 10-15 minutes. Use two forks to shred the meat, discarding any large pieces of fat or the bone.
Serve immediately. Leftovers can be stored in an airtight container in the refrigerator for up to 5 days.
After shredding, return the pork to the slow cooker and toss it with the cooking liquids to keep it moist and add a huge boost of flavor.
Let the pork rest for 15-20 minutes after cooking and before shredding. This allows the juices to redistribute, ensuring every bite is tender and juicy.