Add salt to the water and heat over a low flame
When the water starts to steam (not boil) add the tuna and continue to boil over a low flame
As the broth cooks, some spume will rise to the surface – scoop these out at regular intervals
Keep on simmering on a low flame until there is no more spume
Add onion and curry leaves, if desired
Continue to simmer for at least 10 more minutes
Serve with rice or roshi