Preheat oven to 180 degrees Celcius.
Make a parcel with a piece of double thickness foil. Wipe the mushrooms of dirt and cut into slices. Smear each side of the fish fillet with half the amount of grated ginger. In a small bowl mix the mushrooms with the remaining ginger, garlic, sesame oil and tamari. Lay the mushrooms onto the foil. Wrap the foil parcel securely leaving space above the mushrooms to add the fish. Place in the oven and cook for 15 minutes.
While the mushrooms are cooking, wash and trim the bok choy, and set aside.
With 4 minutes cooking time remaining, heat a non stick frying pan with the sesame oil. Sear each side of the fish for 1 minute each side and season with black pepper. Remove the mushrooms from the oven and carefully open the parcel. Drop the fish onto the top of the mushrooms, secure the parcel again and return to the oven for a further 4 minutes.
Using the same pan cook the bok choy in the remaining sesame oil for 3 minutes until cooked. Remove the fish and mushrooms from the oven and serve with accompanying bok choy.
Note: Buy organic soy (shoyu) or tamari. Both are made from soybeans, water and salt but soy adds wheat to the fermenting process. Some highly processed sauces can contain MSG in the form of hydrolysed vegetable protein and other artificial flavourings/colourings to be avoided.