Before you start, preheat oven to 220°C (or 200°C fan bake). Bring a medium pot of salted water to the boil. Set aside a medium baking dish.
Cook noodles in pot of boiling water on high heat for 10 minutes, until just tender. Drain, refresh under lukewarm water and toss with oil to prevent sticking. Set aside to drain.
Trim mushrooms and scatter in a baking dish in a single layer. Pour over dashi broth, butter and soy sauce. Bake for about 10 minutes. Meanwhile, prep bok choy.
Pat salmon dry, remove any remaining pin bones and cut into 4-5 pieces. Season lightly with salt. Heat oil in a large fry-pan on medium-high heat. Cook salmon, skin-side-down, for about 3 minutes, until skin is crispy but salmon is not cooked through yet. Remove from heat.
Remove baking dish from oven and season broth to taste with soy sauce. Submerge bok choy leaves in broth and place salmon, skin-side up, on top of bok choy. Return to oven and bake for a further 5-7 minutes, until salmon is medium-rare or cooked to your liking.
Prep spring onion and nori.
Serve noodles and broth topped with salmon, spring onion and nori.