Slice the onion into half moon shapes, and place in a gallon-size freezer bag.
Add the pork tenderloin to the bag followed by the barbecue sauce and black pepper. Seal the bag.
Place the bag in the freezer for up to 3 months.
Pull the bag out of the freezer the night before you want to cook the pork.
To cook the pork, dump the entire bag in the slow cooker, and cook on HIGH for 4-6 hours.
After 4 hours, test the pork by trying to shred it with 2 forks; it should easily shred. If not, place it back in the slow cooker for another hour.
Serve the shredded pork on hamburger buns with extra barbecue sauce and any toppings you like.
