For best results, buy a whole fish and have it filleted by the monger. Bring home everything and use the head and bones to make the broth and strain. Cooking time will be longer, for sure, but you'll get the richest flavors.
Grind all the solid ingredients in the spice base list. Pour in the fish sauce and diluted coconut sugar slowly to make grinding easier.
Heat the fish broth in a pot, stir in the spice base, tomato, tied lemongrass and fish sauce to taste. Bring to the boil, lower the heat and simmer for ten minutes.
Slowly pour in the coconut cream and stir. Taste and add more fish sauce, as needed.
Bring to a gentle boil and allow to cook, uncovered, for five minutes.
Stir in the fish slices and cook for another five minutes.
Taste the broth, add more fish sauce, if needed, before stirring in the lime juice.