Heat a pan of water and place a steamer on top.
Create a walled circle with foil to fit the steamer.
Layer bok choi, fish, ginger, lemongrass, sesame oil, and soy sauce onto the foil.
Steam the fish for four minutes inside the steamer.
Cover rice noodles with boiling water for two minutes until soft.
Drain the noodles and combine them with sliced spring onions.
Pound garlic, ginger, chilli, sugar, and herbs into a paste using a mortar.
Mix vinegar, lime juice, and oil into the paste to finish the dressing.
Toss the noodles with the dressing just before serving.
Plate noodles, top with fish, and drizzle with the remaining steaming juices.