Chop the onion finely. Melt 25 g (1 oz) of the butter in a frying pan and soften the onion, without letting it brown, for 5 minutes. Heat the oven to 180°C, 350°F, Gas Mark 4. Put the fish into a buttered, shallow ovenproof dish. Season it with salt and pepper and spoon the onion and butter mixture over the top. Put in the rest of the butter and cover the dish with foil.