Note: If you find this too salty, substitute low salt soy or tamari for the soy sauce.
Preheat the oven to 180°C/350°F.
Cook both types of onions in the oil over medium heat until soft, not brown, around 15 minutes. Combine the sauce ingredients in a bowl. Spread the cooked onions and oil over both sides of the meat. Put the meat in a roasting pan, bone side up. Pour over the cooking sauce. Cover with foil and cook in the oven for 2 hours, basting once.
After the 2 hours, remove the foil and turn the brisket over so the bones are now underneath. Spoon the onions on to the top of the meat and continue to cook, basting every 20 minutes. Cook for 1½ hours until fork tender. Allow to rest for 15 minutes before cutting into 2-bone portions to serve.
Cooking a large brisket: If the brisket is a large one (like the one in my own pics), you will need to add up to an hour of cooking time with the foil on. My test brisket was 3.5 kg and the total cooking time was 4.5 hours. The meat was not soft enough at the end, so I wrapped it in foil, and put it in the oven for an additional hour until the meat was fork tender. If you are starting with a big one, add the hour of cooking before you take the foil off.