Get fillets/steaks of white fish (ideally skinless and boneless) - lay in buttered oven dish.
Pour over them an italian-style tomato pasta sauce, eg a loyd grossman ready-made jar type one (or make your own with fried onions and garlic, tin of tomatoes, seasoning, herbs, bit of wine etc.)
Bake for about 25 mins on medium heat eg gas mark 5-ish.
Check fish is cooked through, give it another 5 if not.
Serve with mashed potato and a green veg such as green beans or broccoli.