Place the mirin in a saucepan and bring to the boil. Stir in the miso paste and caster sugar until they dissolve. Allow the marinade to return to the boil before quickly removing from the heat and leaving to cool.
Place the cod in a bowl or sealable food bag and pour over the cooled marinade. Cover (if it is in a bowl) and leave in the fridge to marinade for 24 hours.
When you are ready to cook the cod, pre-heat the grill to high. Remove the cod from the miso marinade, and place on an oiled baking tray – skin side down. Reserve the marinade. Cook for about 12 minutes until the top is dark golden and the fish tender and cooked through.
While the cod is under the grill, prepare the noodles according to the pack instructions, then drain and set aside.
Heat the oil in a wok. Cut the tenderstem broccoli into 2-3cm long pieces and add to the wok along with the garlic and shiitake mushrooms. Stir fry for a couple of minutes before adding the noodles, pak choi and spring onions.
Stir fry until everything is heated through and then add a couple of large spoonfuls of the miso marinade (discard the rest). Cook for a minute or so for the marinade to boil and coat all the ingredients in the wok, and finish by adding the lime juice and coriander.
Serve the tenderstem broccoli and shiitake mushroom noodles with the miso cod, spooning any sauce from the wok over the top of the fish.