Clean, scale, empty and dry the fish. Make the forcemeat stiff. Stuff the trout with forcemeat, and sew up or skewer the openings. Place in a baking-tin or dish with 2 oz. of butter and bake in a moderate oven (350° F., Gas 4) for about ½ hr., basting frequently. Fry the flour and the rest of the butter together. When the fish is ready, remove it to a hot dish and strain the liquor in the baking-dish on to the flour and butter. Stir until boiling and smooth, then add the capers, lemon juice, anchovy essence and season to taste. Simmer for 2–3 min., then pour over the fish and serve.