You will also need a baking sheet measuring 14 x 11 inches (35 x 28 cm), greased.
To make the pastry, first of all remove a pack of butter from the fridge, weigh out 6 oz (175 g) then wrap it in a piece of foil and return it to the freezer or freezing compartment of the fridge for 30-45 minutes. Then, when you are ready to make the pastry, sift the flour and salt into a large roomy bowl. Take the butter out of the freezer, fold back the foil and hold it in the foil, which will protect it from your warm hands. Then, using the coarse side of a grater placed in the bowl over the flour, grate the butter, dipping the edge of the butter into the flour several times to make it easier to grate. What you will end up with is a large pile of grated butter sitting in the middle of the flour.
Now take a palette knife and start to distribute the gratings into the flour – don't use your hands yet, just keep trying to coat all the pieces of fat with flour. Now sprinkle 2-3 tablespoons of cold water all over, continue to use the palette knife to bring the whole thing together, and finish off using your hands. If you need a bit more moisture, that's fine – just remember that the dough should come together in such a way that it leaves the bowl fairly clean, with no bits of loose butter or flour anywhere. Now pop it into a polythene bag and chill for 30 minutes before using.
To make the filling, place the fish in a medium-sized saucepan with just enough milk to cover, bring to the boil, cover and simmer gently for about 5-10 minutes. Now strain off the milk into a measuring jug, and when the fish is cool enough to handle, flake it into large pieces (discarding all the bones and skin), place in a bowl and set aside.
Next, melt the butter in the same saucepan and stir in the flour. Cook for about 2 minutes over a medium heat, then gradually add 10 fl oz (300 ml) of the milk the fish was cooked in, stirring all the time. Bring the sauce to the boil, simmer gently for 6 minutes, then take the pan off the heat and add the flaked fish, chopped capers, gherkins, parsley and eggs. Season with salt, pepper and lemon juice. Cover and leave until the mixture is quite cold.
When you're ready to cook the pie, pre-heat the oven to gas mark 7, 425ºF (220ºC). Roll out the pastry to a 12 inch (30 cm) square, trimming if necessary. Lift the square on to the greased baking sheet, then place the cold fish mixture in the centre. Glaze around the edge of the pastry with beaten egg, then pull the opposite corners of the pastry to the centre and pinch all the edges together firmly, so you have a square with pinched edges in the shape of a cross. Glaze all over with beaten egg and decorate with any pastry trimmings. Glaze these too and then bake the pie for about 30 minutes or until the pastry is well risen and golden.
