Combine the soy sauce, mirin, sesame oil and sugar in a bowl. Slice each fillet in half length ways. Place in the bowl and turn to coat. Cover and refrigerate for 30 minutes.
Thinly slice the green parts of the spring onions and place in ice-cold water to curl.
Preheat a grill. Place the salmon on a foil-lined baking tray. Grill for about 5 minutes — brushing occasionally with the marinade — until golden. Drain the spring onions and garnish the salmon. Great served with Japanese noodles tossed in a little sesame oil.