1. Preheat oven to 325 degrees Fahrenheit (170 degrees Celsius).
2. Rinse brisket and pat dry.
3. Slice the onion and arrange on the bottom of a roasting pan. Lay the brisket on top of the onions.
4. Season well with salt and pepper. Smear the garlic cloves over the meat. Pour chicken broth around the roast.
5. Combine barbecue-sauce ingredients and pour over the brisket.
6. Cover well and bake for four hours or until fork-tender. Use two forks to pull apart the brisket.