Score the fish and rub lightly with salt and pepper.
Grind the lemongrass, ginger, garlic and chilies into a coarse paste.
Heat the cooking oil and deep fry the fish, turning if not completely submerged in oil, about four minutes per side or until deep golden and crisp on the outside.
Drain the fish and keep warm.
Pour off the oil from the pan leaving only about two tablespoonfuls.
Over medium heat, sauté the ground spices until fragrant, about a minute.
Pour in about two tablespoonfuls of fish sauce and two tablespoonfuls of water.
Add a generous pinch of sugar.
Stir. Taste and adjust the seasonings until you're happy with the balance.
Return the fish to the pan. Lower the heat, cover and cook for one minute per side.
Scoop out the fish and move to a serving plate.
Pour the ground spices and any remaining liquid over the fish.
Thinly slice the mint leaves and sprinkle over the fish.