Thai diners like their fish completely cooked and will steam this over a furious heat. But as a concession to modern styles of cooking, I suggest you steam it over a moderate heat to ensure a tender texture - steaming over a rolling boil can be harsh and may toughen the flesh. Some Thai cooks fill the cavity with a few stalks of lemongrass, a pandanus leaf or a few coriander roots.
1 x 400g fish - eg, sea bass, barramundi, John Dory, snapper or bream - gutted and scaled
1 banana leaf, optional
Chopped coriander, to serve
Chilli and lime sauce
1-2 coriander roots, cleaned
Pinch of salt
2-3 garlic cloves, peeled
3-5 bird's eye chillies (scuds), or more, if you wish
1 tbsp white sugar
3-4 tbsp lime juice
1-2 tbsp fish sauce, to taste
Rinse the fish well and pat dry with paper towel. Score the fish by making three or four diagonal slices on each side. This helps the fish cook faster and more evenly.
Next, make the sauce. Using a pestle and mortar, pound the coriander roots to a fine paste with the salt.
Add the garlic and chillies and continue pounding to a coarse paste. Season with the sugar, lime juice and fish sauce. The sauce should be hot, sour, salty and more than a little sweet - but this can be altered to taste. Transfer the sauce to a small bowl and set aside.
Place the fish on a banana leaf or heatproof plate and then into a steamer. Steam over simmering water for about 15-20 minutes or until cooked - the scored flesh should be opaque right to the bone.
Spoon the chilli and lime sauce over the fish and serve sprinkled with chopped coriander.