Put the salmon fillet in a shallow dish. Add the Kikkoman soy sauce, sesame oil, sesame seeds and rice vinegar. Coat the fish well and marinate for at least 15 minutes (up to 12 hours).
Mix the dressing in a small bowl and set aside.
Heat a griddle pan. Add the salmon, skin side down first, and cook for approx. 3 minutes on each side (depending on thickness), then move it to a plate to rest. Meanwhile, dip the shiitake in the salmon's marinade and add them to the griddle pan too. Fry for approx. 3-5 minutes until tender.
Put the broccoli in a steamer or boiling water and steam or cook for 2-3 minutes, until tender. Transfer them to two serving plates.
Discard the skin of the salmon then flake the flesh into bite-sized pieces. Arrange these on top the broccoli, then add the shiitake and scatter the spring onion pieces all over the dish. Drizzle with the dressing and serve.