Stuff the seabass cavity with lemongrass, pandanus leaf and kaffir lime leaves, (you can even poke some lemongrass out through its mouth if you like) and pin closed with skewer.
In a bowl, make a fairly thick paste by mixing the rice flour (or corn flour) with two tablespoons of water.
Rub the skin of the fish on one side with the paste then pour over the salt generously, patting it down to stick.
Leave for 5 mins, turn over the fish and then repeat the paste and salt on the other side.
Grill fish on a medium temperature BBQ for around 6-8 minutes on each side or until cooked. You can use a griddle pan to do this if you don't have a BBQ.
In the meantime, make the dipping sauce by pounding the coriander roots or stalks, garlic, chillies and galangal (if using) in a pestle and mortar until fairly smooth, then mix in the lime, fish sauce and sugar (plus mandarin juice if using).
Serve the fish on the banana leaf with a small bowl of the dipping sauce alongside.