Clean and scale the fish. Remove the beards of the oysters and simmer for 15 min. in a little fish stock or water. Cut the oysters into small pieces, but do not cook them; also cut the anchovies into small pieces. Mix together the breadcrumbs, oysters, anchovies, parsley, finely-chopped shallot and seasoning; add the egg yolk, the liquor of the oysters and the stock in which the oyster beards were simmered. Put the forcemeat inside the fish and sew up the opening; brush over with egg and coat with breadcrumbs. Place in a baking-dish and cook gently for about 1 hr., basting frequently with hot butter. To make the sauce: melt the butter in a saucepan, stir in the flour, add the stock and stir until the sauce boils.
Simmer for 2–3 min., then add the mustard, lemon juice, Worcester sauce and the liquor (strained) in which the fish was cooked. Garnish the fish with cut lemon and parsley, and serve the sauce in a sauceboat.