Cook the noodles according to the pack instructions. Meanwhile, heat a griddle pan until smoking hot. Rub the oil over the tuna steaks and season well. Cook in the pan for 1–2 minutes on each side, or until charred on the outside but still rare inside.
Drain the noodles, cool under cold running water and drain again, then divide between serving bowls.
Cut the tuna into slices and toss together with the cucumber and spring onions. Mix together the soy sauce, mirin, lime juice and sugar until the sugar has dissolved. Pour over the noodles and sprinkle with sesame seeds to serve.