Make the béchamel sauce in the usual way. Skin the fillets of fish. Smear half the butter over the base of an ovenproof dish. Tuck in the ends of the fish fillets and arrange in a single layer on the dish. Season with salt and freshly ground pepper. Put the washed mussels into a stainless steel saucepan, add the wine, cover and place on a low heat just until the mussels open, 2–3 minutes. Take the mussels from the shells, remove the beard and keep the mussels aside. Pour the liquid over the fish, cover with tinfoil and bake in a moderate oven, 180°C/350°F/Gas 4, until the fish is almost cooked, approximately 10–15 minutes.
Meanwhile sauté the mushrooms in the remaining butter in a hot pan, season with salt and freshly ground pepper. Strain off the cooking liquor and add it to the béchamel. Bring it to the boil. Make the liaison by mixing the egg yolk with the cream, add some of the hot liquid to the cold liaison and then mix into the rest of the sauce; taste and correct the seasoning.
Sprinkle the mushrooms and mussels over the fish in the serving dish and coat with the sauce (may be made ahead to this point). Return to the oven (160°C/325°F/Gas 3) for 15–20 minutes or until the fish is hot and the sauce is golden on top.
© 2001 Darina Allen. All rights reserved.