Sauce.
To one gill of boiling water allow as much milk; stir into this, while boiling, two tablespoonfuls of butter, added gradually, a tablespoonful of flour wet up with cold water, and, as it thickens, the chopped yolk of a boiled egg and one raw egg, beaten light. Take directly from the fire, season with pepper, salt, a little chopped parsley and the juice of a lemon, and set, covered, in boiling water, but not over the fire, for five minutes, stirring occasionally. Pour part of the sauce over the fish when dished; the rest in a boat. Send around mashed potatoes with it.