Stir together all of the rub ingredients, then coat the brisket thoroughly on every side; refrigerate it for 30 minutes to 24 hours if desired. The seasoning should form an even layer over the beef, with no bare patches around the edges.
Mix the BBQ Sauce ingredients directly in the slow cooker, then nestle the brisket into the sauce with the fat side facing up. The sauce should reach around the meat and the lid should sit securely on the cooker.
Cook on low for 8 hours for a 1.5 kg / 3 lb brisket or up to 10 hours for a 2 kg / 4 lb brisket. A fork should slide into the center and twist with almost no resistance when the brisket is tender.
Transfer the cooked brisket to a baking tray, keeping the liquid in the slow cooker for the sauce. The meat will be very soft, so support it from underneath rather than gripping one end.
Pour the slow cooker liquid into a saucepan and simmer it over medium-high heat until it reaches a syrupy consistency. Watch for active bubbles that become glossy and slower as the sauce thickens; it will continue to thicken as it cools.
Drizzle the brisket with olive oil and roast it at 200°C/390°F for 15 minutes, until browned spots form. The exterior should sizzle and darken while the center remains moist.
Baste the brisket generously with the reduced sauce, return it to the oven for 5 minutes, then baste again and cook for a final 5 to 10 minutes. The glaze should look dark, shiny, and caramelized rather than wet or runny.
Rest the brisket briefly, then slice it thinly across the grain and serve with the remaining BBQ sauce. Resting helps the juices settle so they stay with the slices instead of pooling across the cutting board.