Mix miso paste and Mirin. Rinse fish and pat dry. Salt and spread the miso sauce all around with a knife. Covered fish and marinate in the refrigerator for about 3 hours.
Clean and wash the spring onion, halve lengthwise and cut into thin strips. Place in ice-cold water. Clean the mushrooms, wash them briefly if necessary and cut in half. Clean and wash the zucchini, cut in half lengthwise and cut into slices.
Heat 1 tablespoon of oil in a frying pan. Fry the mushrooms on a high heat for 3-4 minutes. Add zucchini and fry for 3-4 minutes.
In the meantime, spread the miso sauce on the fish with wet hands. Heat 1 tablespoon of oil in a second coated pan. Fry the fish for about 2 minutes on each side. Season vegetables with soy sauce, pepper, 1 pinch of sugar and salt if necessary.
Serve fish on top. Drain the spring onion and sprinkle the fish on top. Rice tastes good with it.