Bring the pot to a gentle simmer.
Add the lemongrass, onion, ginger and pineapple.
Simmer the broth over low-medium heat for about 3 hours. During this time, keep skimming the surface of the broth to remove any impurities and excess fat to ensure a clean and clear broth, otherwise the broth will turn cloudy. If a lot of the broth has evaporated, top up with some boiling water so that the water remains constantly at the same level (i.e. just covering the meat).
After 2 hours, add the sugar, salt, fish sauce and MSG/beef stock cube/Bun Bo Hue stock cube.
After 3 hours, or when the meat falls off the bones easily and the beef brisket is very tender, remove all of the meat and bones to a large bowl.
Remove and discard the lemongrass, onion, ginger and pineapple.
Strain the broth through a fine sieve or a muslin cloth, and into a clean stockpot.