More-ish fish dish

Preheat the oven to 180 ºC (350 ºF). Grease an ovenproof dish with margarine.
Arrange the hake fillets in the prepared dish. Season both sides with salt and pepper to taste.
Bake in the oven for 20 minutes or until the fish is nearly done. Pour off the liquid, but reserve it.
In the meantime, sauté the onion in a little oil until soft. Add the mushrooms and stir-fry until pale brown.
Spoon over the fish and sprinkle with parsley.
Add enough milk to the reserved fish liquid to make up 250 ml.
Make a white sauce with the butter, cake flour and flavoured milk. Add the sherry and 50 ml grated Cheddar cheese.
Season to taste with salt, pepper and paprika. Stir until the cheese has melted.
Pour the sauce over the fish.
Sprinkle with the remaining Cheddar cheese.
Bake for another 15 minutes or until the fish is completely done.
Serve with rice and a green salad.