Arrange fish in a single layer on an oiled, shallow baking dish.
Heat oil in a frying pan and saute onion until translucent.
Add tomato sauce, celery, garlic, herbs, salt and pepper. Mix until well combined.
Spoon sauce over fillets.
Cover and bake 25 minutes.
NOTE: If you dont have a baking dish with a cover you can cook the fish is an aluminium foil parcel. Lay foil on your baking tray. Spray with oil. Spread on the fish and top with the sauce. Lay another layer of foil over the top and firmly fold all of the foil's edges together to enclose the fish. Alternatively you can cook the fish in individual foils and serve in the foil parcel.