Mix together all the ingredients for the bbq sauce until well combined. Pour the bbq sauce into the body of the slow cooker.
Add the sliced onion to the bbq sauce, then place the spice rubbed brisket on top.
Cook on low heat for 8-10 hours or until beef is tender and easily shredded with a fork.
Heat the oven to 200 C (390 F ). Rub the cooked brisket with a little of the bbq sauce, then . the vegetable oil and place on a roasting pan. Cook for 10 minutes or until the brisket turns a deep golden brown caramelised color. Meanwhile, turn the heat of the slow cooker up to high, and cook the bbq sauce until it reduces and thickens slightly, 10 minutes should be enough.
Once brisket is out of the oven, brush all over with more bbq sauce, and serve with the remaining bbq sauce, either sliced or shredded.