In a small bowl, add all the ingredients for the dry rub and stir to combine.
Use paper towels to pat the pork shoulder dry and then use your hands to rub the meat with the dry rub until evenly coated. Cover and leave in the fridge to marinate for a few hours.
Spray a medium sized frying pan with the cooking spray. Over a medium-high heat, add the onions and garlic, stirring frequently, until the onion is nice and soft. Transfer to a large slow cooker and add the water.
Place the pork shoulder on top of the onions, fat side up. Cover and cook on low for 10-12 hours
Turn slow cooker off and let the roast rest, covered, for 30-45 minutes.
Use two forks to pull the meat apart. You can then stir some of the cooking juices or just serve with caramelised onion chutney or your favourite sauce/gravy.