Place frozen pork shoulder in slow cooker with 1 bottle of BBQ sauce and cook on low for 5-6 hours (or high for 3-4 hours). You want pork to be cooked through and tender.
Remove pork from slow cooker and use 2 forks to shred the meat. Do not discard the liquid in the slow cooker
Place shredded pork pack in slow cooker and stir into the sauce in the slow cooker. And cook on low for an additional 1 hr
Toast hamburger buns and top with pulled pork, grated cheese and onions, if desired.
Serve warm and enjoy!