In the oven dish in which you intend to cook the dish, saute the mushrooms and onions with some of the oil. On top of the sauteed products, arrange the fish fillets, seasoned as desired.
Dissolve the mushroom soup in the amount of water listed on the package and pour it over the fish, pour the remaining oil in. Bake in a preheated oven for about 30 minutes.
Serve with a side of sauteed vegetables or fresh salad.