2.5 cm (1 inch) piece of fresh root ginger, cut into thin matchsticks
Slice the spring onions very thinly lengthways and place in a bowl of cold water and ice cubes. Set aside in the fridge.
To make the marinade, disolve the sugar in the soy sauce in a small bowl, then stir in the lemon grass, lime leaves and ginger.
Score small, shallow slits into the skin side of the fillets and place them in flat dish. Pour the marinade over and rub it into the flesh. Set aside to marinade for 15 minutes.
Meanwhile, heat the oil in a deep-sided frying pan over a medium heat. Fry the garlic for 2 minutes, then pour in the sake. Bubble to almost evaporate, then add the fish, skin side down, and any marinade. Cover with a lid and steam for 6–8 minutes until the fish is just cooked and the flesh flakes easily.
Cook the soba noodles in a large pan of boiling water for 7–8 minutes, or according to the packet instructions, then drain.
Remove the fillets from the pan, tip the noodles into the pan and toss to coat in the cooking juices. Pile the noodles on to serving plates and place the sea bass on top. Drain the spring onions and sprinkle over each fillet with the sesame seeds.