Set the flame of your stove or burner to high heat then put a wire cooling rack over it. On the rack, put ginger and onions. Turn them over from time to time for around 10 to 15 minutes. Do this until their skin is a little bit blackened. Set them aside.
Next is to make the broth. First, put the bones into the pot immersing them with cold water. Turn on the heat of the stove to high, bringing the pot into a boil. Let the pot boil for about 5 minutes so that all impurities from the bone are released. After 5 minutes, turn the heat down and tip the pot over to get the bones. Rinse them under warm water to remove any debris.
Using another stockpot or after cleaning the one you just used, put the bones back with enough cold water so that they are immersed. Just like before, bring it to boil, so that all impurities are released. In this step, skim the impurities off on the water’s surface for the 15 minutes you let the bones boil. This way, you can get a clean, clear broth for the soup. When the broth is relatively clean, reduce the heat to a low simmer.
Next, add the ingredients – charred onions and ginger, the brisket, fennel seeds, star anise, cloves, cinnamon, coriander, salt, sugar, and fish sauce.
With all these inside the pot, let it simmer gently for around 1 to 2 hours without covering it.
Once the brisket is tender, you can remove it and set it aside, letting it cool down using cold water. Drain before refrigerating it to use for later.
As for the broth, let it simmer for another 3 to 4 hours. Add more water as much as needed to make sure the bones remain immersed.
When you are done simmering the broth, take a fine strainer to get the spices and bones. The broth is likely too little now after too much boiling, so adjust it to become 4 liters while also removing oil from the surface. Taste and adjust the seasoning with rock sugar, salt, and fish sauce if you want to.