 In place of a pastry casing enclosing the pie, a topping of mashed potatoes (sometimes with cheese or vegetables such as onions and leeks added) is used to cover the fish during baking.The dish is sometimes referred to as "fisherman's pie" because the topping is similar to that of shepherd's pie, in that it uses mashed potatoes.The royal seafood tradition of England started in the time of Henry I, crowned in 1100, when cooks rolled crust over an annual Christmas lamprey pie.