Soak a bamboo skewer in cold water for 30 minutes. In a mixing bowl, combine the lemongrass, dill and spring onion. Stuff the mixture into the cavity of the fish, then secure the opening using the bamboo skewer. Rub the whole fish with all the sea salt, coating it well. Heat a chargrill pan or barbecue chargrill plate to medium-high. Chargrill the fish for 10–15 minutes on each side, or until cooked through (the scales and skin should peel easily).Meanwhile, combine the tamarind dipping sauce ingredients in a small bowl and mix until the sugar has dissolved. Before serving the fish at the table, peel away and discard the scales and skin. Serve the fish hot, with sticky rice and the dipping sauce.
Note: To make tamarind water, soak 100g tamarind pulp in 400ml boiling water. Break it up a little with a whisk, then leave until cool enough to handle. Using your hands, break the mixture into a thick paste. Pass the mixture through a sieve; you should get about 375ml tamarind water.