This is the fish dish every Thai seafood restaurant is judged on. The fish is steamed plainly over lemongrass, and all the noise comes from a raw sauce of lime juice, garlic, chilli and fish sauce poured over it at the last second.
Because the sauce is never cooked it stays sharp and alive, which is what cuts through the richness of the fish. Use more garlic than feels sensible and taste the sauce before it goes on; it should make you flinch slightly.
