Make the teriyaki sauce. Put all the ingredients in a small saucepan and stir over a medium heat until the sugar has dissolved. Increase the heat a little and simmer for 5 minutes until thickened. Set aside to cool.
Meanwhile, rub the teriyaki sauce over the salmon fillets and arrange them in an ovenproof dish. Cook under a preheated grill for 4–5 minutes on each side, basting occasionally. Remove and set aside.
Heat the sesame oil in a frying pan and stir-fry the spring onions for 2 minutes. Add the stock, soy sauce, miso paste and mirin or sugar, stirring to dissolve. Simmer gently and add the noodles and pak choi and cook for 2 minutes until the leaves have wilted.
Serve immediately topped with the grilled salmon.