Preheat your oven to 150°C/300°F (fan-forced).
In a small bowl mix together the spices, salt and pepper.
Rub the beef with about 1½ tablespoons of the spice blend to coat it and place in a medium sized Dutch oven, casserole dish (with a lid) or roasting pan. Place the remaining spice mix in a small jar for your next brisket.
Add the garlic, onion and pour over the stock (it should cover or almost cover the meat).
Cover with a lid or foil wrapped tightly to seal.
Cook for about 2 hours, turn the meat and cook for another 2-3 hours (total 4-5 hours cook time). It is cooked when the meat easily falls apart when pulled at.
Remove the lid or foil and turn the oven up to 200°C. Pull the meat apart and cook for approximately 30 minutes to caramelise and thicken the sauce.
To make your tahini sauce, combine all of the tahini sauce ingredients together. Add the water tablespoon by tablespoon until you have a slightly runny sauce that can be drizzled over the meat.