Place onions in the bottom of a 5-quart slow cooker. Season pork butt with salt and pepper. Place in slow cooker on top of onions, fat side up.
In a medium bowl, stir together barbeque sauce, mustard, tomato paste, chicken broth and lime juice. Pour over pork. Cover and cook on LOW setting for 6 to 8 hours.
Remove pork and let rest for 5 to 10 minutes.
In a large bowl whisk together sour cream, mayonnaise, lime juice, and hot sauce. Add coleslaw blend and toss to coat. Season with salt and pepper. Place in refrigerator for 1 hour before serving. Serve on top of pork sandwich.
Slice pork and serve on buns with spicy coleslaw.