Marinate the brisket in the liquid smoke in a plastic bag overnight. Place onion slices in a single layer in the crock of the slow cooker. Place the meat (fat side up) on top of the sliced onion pieces in the slow cooker.
Sprinkle salt and pepper over the brisket. Spread the minced garlic over the top of the meat pressing it in. Pour the Worcestershire sauce over the top of the meat.
Cover the slow cooker and cook on low for 8 to 10 hours. One hour before the brisket is done brush the barbecue sauce over the meat, recover and continue to slow cook for the remaining time.
Remove the brisket from the crock pot and place it on a platter to rest for 10 to 15 minutes before slicing. Slice the brisket against the grain.Serve as desired.