Make a paste by blending together the chillies, shallots, garlic, lemongrass, ginger, soy sauce and sugar in a blender or coffee grinder, or use a pestle and mortar. Heat the oil in a small frying pan and cook the paste for 2 mins until it has darkened slightly and you can really smell the spices. Season with a little salt if you like, then set aside.
Heat the grill to medium. Put the fish onto a grill pan, then use the back of a teaspoon to smear the paste all over, making a thin layer covering the whole fish. Pop under the grill and cook for 10 mins until the fish flakes easily. Serve with rice and steamed bok choy. Sprinkle chives and coriander on the fish to serve, if you like.