Combine all ingredients in a 3-1/2 quart crockpot. Add salt and pepper to taste.
Cover and cook on LO heat for 8 hours.
When meat is tender, use two forks to shred it. Use a slotted spoon to remove the onion pieces from the broth in the pot if desired. Return the meat to the broth. Use the slotted spoon to spoon the meat onto the buns.