In a small bowl, whisk together the barbecue sauce, chili seasoning, Worcestershire sauce, garlic and lemon juice. Add onion, and stir to combine. Pour the mixture evenly over the brisket.
Cover the brisket, and cook on high until the meat is tender and falling apart, about 5 to 6 hours.
Reduce the slow cooker to low heat. Carefully transfer the beef to a large platter. Allow beef to rest for 5 to 10 minutes.
Using a pair of forks, shred the meat completely. Return the shredded beef to the slow cooker, and stir to coat with the warm juices. Replace the lid and allow the meat to reheat, about 10 minutes.