Preheat the oven to 350°F (175°C).

Lightly oil two pieces of aluminium foil and lay each piece of fish in the middle of one of the pieces of foil.

Seal the foil into parcels and place on a baking tray.

Bake for 15 to 20 minutes depending on the thickness of the fish. Fish is done when it is opaque and flakes easily with a fork.

Remove the fish from the oven and keep sealed in the foil packets.

Meanwhile, while the fish is cooking, melt the butter over medium heat in a large frying pan.

Add the onions and cook until they begin to soften.

Add the mushrooms and cook until they begin to pick up a bit of colour.

Sprinkle the flour over the onions and mushrooms and stir in.

Cook for a minute or two, stirring constantly.

Add half a cup of the chicken or fish stock, a bit at a time, stirring after each addition and allowing the sauce to thicken.

Add the spinach and stir through the sauce.

As the spinach begins to wilt, start to add the cream, a bit at a time, stirring after each addition.

If the sauce becomes to thick, add a bit more chicken or fish stock.

Taste the sauce for seasoning and add salt and pepper if necessary.

Remove the fish from the foil packages, place on a plate and serve the spinach and mushroom sauce over top.