Add the onion into the Crock-Pot. Combine the ingredients for the spice rub in a small bowl. Pat the pork shoulder dry with a paper towel. Rub the spice mixture into the pork and set into the Crock-Pot. Drizzle with 1 cup of the barbecue sauce. Cover and cook on low heat for 7-8 hours.

Remove the pork from the Crock-Pot and shred. Strain any of the liquid remaining in the slow cooker through a fine mesh strainer or cheesecloth. Place the pork back into the empty Crock-Pot, along with the remaining cup of barbecue sauce. Pour in 1/4-1/2 cup of the strained liquid and stir. Cook on low for an additional 30 minutes. Serve warm.