Wash the fish well and pat dry on kitchen paper. Place in a shallow ovenproof dish, sprinkle with salt and leave to stand for about 45 minutes.

Preheat the oven to 200C, 400F, Gas Mark 6 and butter a shallow baking dish.

Rinse the fish, dry on kitchen paper and lay in the prepared ovenproof dish, skin side down. Sprinkle with the onion, bacon, cheese and breadcrumbs then pour the melted butter over fish.

Pour half the sour cream over the top and bake for 30 minutes, basting with the pan juices from time to time. If the pan seems to be drying out, add a little white wine or fish stock.

Add the remaining sour cream to pan but don't pour it over the fish, and continue to bake for a further 15 minutes. Serve hot.