Heat oil. Cook onion and garlic until transparent. Add mushrooms, oregano or marjoram and pepper. Stir until mushrooms soften slightly.
Cut two 30cm square pieces of foil or baking paper and lay one of the fillets in the centre of each piece. If using foil, grease lightly.
Arrange half the filling mixture on top of each fillet. Top with remaining fillet. If using whole fish, place the filling mixture into the cavity of the fish. Top with extra herbs if desired.
Roll the foil over the fish like a parcel. Place on a baking tray with edges of foil downwards. Bake in a 190°C oven for 20 - 30 minutes. Serve with oven-baked chunky chips or jacket-cooked potatoes.