Soak breadcrumbs in milk until moist, mix with lemon juice and rind, half the chopped onion plus most of the parsley and seasonings. Use to stuff fish, scattering any leftover over the base of a deep, round 23cm or similar pie dish.
Arrange fish in the dish with their heads sticking up towards outer edge, scatter the remaining onions and parsley, chopped bacon and chopped eggs in between the fish, season well and carefully pour in the cider.
Roll out pastry to fit the top of the pie dish, use trimmings to make a pastry rim to help the lid to stick to the top.
Place pastry on top of pie and make slits in top where the heads are and then poke them through so they stand up. Press edges firmly together to deal, trim and brush top with beaten egg. Make a couple of slits or holes in centre to allow steam to escape and bake for 40-50 mins until pastry golden brown. Scatter with chopped parsley to serve.