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 enclose the contents 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. A separate tradition of Lenten fish pie required Yarmouth cooks to send the king two dozen pies containing 100 herrings. The customary gifts of fish in crust prevailed in 1530, when the prior of Llanthony, Gloucester, baked eels and carp into a pie for Henry VIII. The presentation of the royal eel pie continued in 1752, when bakers sent one to the Prince of Wales, and again during Queen Victoria's reign."