Cut roast in half. In a large skillet, brown roast in oil; place in a 5-qt. slow cooker. Add the onion, 1/2 cup teriyaki sauce and pineapple juice. Cover and cook on low for 7-9 hours or until meat is tender.
Remove roast; set aside. In a small bowl, combine flour and remaining teriyaki sauce until smooth; stir into cooking juices. Cover and cook on high for 30-40 minutes or until thickened.
Shred meat with two forks; return to the slow cooker and heat through. Spoon 1/2 cup onto each bun; top with a pineapple slice.