Place onions and garlic in an even layer in the bottom of a slow cooker and pour in water or chicken stock.
Combine brown sugar, chili powder, paprika, salt, and pepper in a small bowl. Rub the spice mixture all over the pork and place the meat on top of the onions and garlic.
Cover and cook on LOW until the meat is tender, about 6 to 8 hours. Remove the pork and shred the meat with two forks. Skim and discard fat from the cooking liquid and return shredded pork to the slow cooker. Stir in the barbecue sauce until it is well coated and cook for one more hour on LOW.
Meanwhile, whisk together buttermilk, mayonnaise, mustard, vinegar, and sugar in a large mixing bowl. Toss in coleslaw mix and season with salt and pepper to taste. Refrigerate for at least 15 minutes before serving with pulled pork sliders.