1. Preheat oven to 140°C (275°F, fan).
2. Season the brisket with salt and pepper. Heat oil in a large casserole or Dutch oven over high heat. Brown the brisket on all sides for about 3-5 minutes per side until well caramelized. Remove and set aside.
3. Reduce heat to low. Add sliced onion, season with salt and pepper, and cook gently for 5 minutes until softened, stirring regularly.
4. Mix smoked paprika, cinnamon, cayenne, dark brown sugar, honey, and water in a small bowl. Spread half of this mixture evenly over the brisket.
5. Add crushed garlic to the onions and cook for 30 seconds. Stir in chopped tomatoes, beef stock, Worcestershire sauce, soy sauce, and brown sauce. Bring the sauce to a boil.
6. Place the marinated brisket on top of the sauce, cover with a lid, and transfer to the oven. Cook slowly for 3 hours, checking occasionally and adding water if the sauce reduces too much.
7. Remove the brisket from the oven. Stir the sauce and spread the remaining spice mixture on the brisket. Sprinkle with light brown sugar, salt, and pepper.
8. Increase the oven temperature to 200°C (400°F, fan) and cook the brisket uncovered for 20-30 minutes to develop a dark, sticky caramelized crust.
9. Remove from oven, sprinkle with chili flakes if desired, and let rest briefly. Shred the meat using two forks right in the sauce.
10. Serve warm with rice, potatoes, salad, or a side of sweet chili jam.