Finely dice the onion and garlic clove and sauté in a little fat until translucent. Add the cleaned and finely sliced mushrooms and let simmer for a few minutes. Top up with the vegetable stock and white wine and let it simmer a little.
Finely slice the spring onion and basil leaves and add to the sauce. Also add the creme fraiche and season with salt and pepper to taste. Possibly thicken a little.
Finally quarter the tomatoes and add them to the sauce.
Place the fish in a baking dish and pour the sauce over it. Put the whole thing in the oven for 30 minutes at 200 degrees.