Arrange onion slices on bottom of crock pot, cut roast in half and place on top of onions. Sprinkle with garlic, salt and pepper, pour in broth and bourbon over the top.
Cover and cook on low for 8-10 hours until beef is tender
Transfer beef to a tray, pour liquid in a deep bowl. Trim fat from beef. Using 2 forks, shred beef
Return beef to crock pot, use a slotted spoon to lift onions from the liquid and return onions to crock pot.
Skim fat from crock pot, measure out ½ cup liquid and pour over beef, discard extra liquid. Stir in barbecue sauce