Wipe pot clean, bring 3.5 litres / 3.75 quarts water to boil.
Add bones and brisket, onion, ginger, Spices
Add onion, ginger, Spices, sugar and salt - water should just barely cover everything.
Cover with lid, simmer 3 hours.
Remove brisket (should be fall-apart tender), cool then refrigerate for later.
Simmer remaining soup UNCOVERED for 40 minutes.
Strain broth into another pot, discard bones and spices. Should be about 2.5 litres / 2.65 quarts (10 cups), if loads more, reduce.
Add fish sauce, adjust salt and sugar if needed. Broth should be beefy, fragrant with spices, savoury and barely sweet.