- Place pork tenderloin in a glass dish. Stir together hoisin sauce, rice vinegar and pineapple juice in a small bowl. Pour 1 cup of marinade over pork. Cover and refrigerate 4-6 hours. Store remaining marinade in the refrigerator.
- Spread sliced onion in the bottom of the slow cooker. Remove the pork from the marinade and place onto onions. Discard the marinade the pork was in. Pour the reserved marinade over the pork and then cover and cook for 6 hours on high or 8 hours on low.
- Once cooked, shred pork on a cutting board with two forks and return to slow cooker. Toss in sauce and then serve hot on buns or over rice.