In a large mixing bowl, combine Worcestershire sauce, chili powder, minced garlic, celery salt, pepper, and optional liquid smoke. Coat the beef brisket halves evenly in this flavorful marinade. Cover the bowl tightly with plastic wrap and refrigerate for at least 8 hours, ideally overnight.

When you're ready to cook, remove the brisket from the refrigerator. Place the marinated brisket into a slow cooker, adding the beef broth and bay leaves. Secure the lid tightly and set your slow cooker to low. Let the brisket cook for 6-8 hours.

About 30 minutes before the brisket is done, heat a skillet over medium heat and add canola oil. Sauté the chopped onion until it’s soft and translucent, about 5 minutes. Add the minced garlic and cook for another minute until fragrant. Stir in ketchup, molasses, cider vinegar, remaining chili powder, and ground mustard, mixing thoroughly to create a rich barbecue sauce.

Once the brisket is fully cooked, carefully transfer it to a cutting board. Skim off excess fat from the slow cooker juices and mix those juices into the prepared barbecue sauce. Return the brisket to the slow cooker, pour the sauce over it, and cover. Set the cooker on high for an additional 30 minutes.

After the final cooking time, remove the brisket from the slow cooker and let it rest for at least 10 minutes before slicing. Serve with extra barbecue sauce on the side.