Whisk BBQ sauce in the slow cooker, then add brisket fat-side up. Cover and cook on LOW: ~8 hrs for 3 lb (1.5 kg), up to ~10 hrs for 4 lb (2 kg), until fork-tender yet sliceable.
Move brisket to a sheet pan. Pour slow-cooker juices into a saucepan and simmer over medium-high 8–12 min, stirring, until glossy and thick enough to coat a spoon (it will thicken more as it cools).
Caramelize in the oven Preheat to 390°F / 200°C. Roast brisket on the sheet pan 15 min until lightly browned. Brush with reduced sauce, roast 5 min; brush again and roast 5–10 min more, until caramelized.