To make the teriyaki sauce, combine the tamari, mirin, sake and caster sugar in a bowl and stir to combine.
Heat vegetable oil in a non-stick frying pan over high heat. Add the salmon and cook, turning, for 2-3 minutes until just cooked. Turn off heat and pour over the teriyaki sauce.
Meanwhile, cook soba noodles according to packet instructions (usually around 3 minutes). Drain and rinse thoroughly. Mix through sesame oil, sesame seeds and spring onion.
Cook pak choy in leftover noodle water for 1-2 minutes until leaves are just wilted and stems have softened.
Divide noodles and pak choy onto serving dishes, top with salmon and extra spring onion. Pour any remaining sauce over the salmon and noodles. Sprinkle with sesame seeds.