Mix together the prawns, lime rind, ginger and chilli in a bowl. Tip into a steamer basket and lower into a shallow saucepan of boiling water so the prawns are not quite touching the water. Cover and steam for 3–4 minutes or until the prawns turn pink and are cooked through. Alternatively, cook the prawns in a bamboo or electric steamer. Remove from the pan and leave to cool.
Meanwhile, whisk together all the dressing ingredients in a small bowl.
Cook the noodles in a large saucepan of boiling water for 6–7 minutes, or according to the packet instructions, until tender. Drain well and rinse under running cold water. Toss with the dressing.
Divide the noodles between 4 bowls, scatter over the spring onions and prawns and serve immediately.