Boil a kettle of water and tip the noodles into a large heatproof bowl. Cover them with boiling water and leave for 5 mins. Meanwhile, mix the soy sauce with the garlic, ginger and 3 tbsp of water to make a dressing.
Drain the noodles, rinse under the cold tap, then drain them again. Tip into a bowl and toss in the cucumber, spring onions, half the coriander and half the dressing. Top each portion of noodles with mackerel, drizzle the remaining dressing over and sprinkle with the rest of the coriander. Serve.