Create a sauce using the first three ingredients.
Peel and thinly slice the ginger.
Finely chop the onion and mushrooms.
Heat a little oil and fry the cod.
In a separate frying pan, fry the ginger and onion. Add the mushrooms and cook for 1-2 minutes.
Add the sauce to the vegetables and cook for a further minute.
Serve the fish on the mushroom sauce.