Heat a large frying pan, add in olive oil and then the brisket fat side down. Cook for 5 minutes to render the fat. Continue cooking, turning, for 8 minutes or until browned on both sides. Transfer to a chaffing tray.
Add the BBQ sauce to the frying pan and simmer for 3 minutes or until reduced by half. Add the HP Sauce and 3 cups water, stirring to combine. Bring to a boil and pour over the beef.
Cover with foil and place in the oven at 160oC for 4 hours, turning the brisket halfway, until very tender. Transfer brisket to a tray, cover and keep warm.
Pour sauce into a saucepan on a high heat and simmer liquid for 15-20 minutes or until thick and saucy.
To make slaw combine the cabbage, carrot, onion relish, chopped parsley, tomato sauce and mayo.
Slice the brisket. To construct the sandwich, use two pieces of sourdough per portion serve and approximately 170g sliced brisket. Top with some of the HP glaze, sliced beetroot and Slaw. Serve with beer battered chips or wedges.
Tip: Secondary cuts are full of flavour, try this recipe with other cuts such as beef hanger steak or lamb neck.