In a steamer put the fish fillets in a bowl. Scatter galangal, fresh lemongrass, kaffir lime leaves, fish sauce, red chillies, and steam till it cooks through.
Meanwhile, heat the oil in a pan, brown the garlic and add spring onions for 30 seconds.
Pour the light soya sauce and fish sauce and add the brown sugar. Add lime juice right at the end.
Pour this sauce over the fish when it is steamed. Also pour the leftover liquid from the bowl.
Serve while piping hot.