In a large slow cooker, put in half the onions and pineapple. Place the marinated pork loin filet on top. Cover pork with remaining pineapple and onion.
Cook on low for approximately 6 hours or high for approximately 4 hours (until pork is fork tender).
Once done, use two forks to shred the pork filet right in the crock pot. The pineapple and onions should combine well with the pork as you shred.
Place a generous amount of pulled pork on each Hawaiian roll, top with desired amount of Hawaiian BBQ sauce, and serve!