To make the pastry, sift the flour and salt into a large bowl. Drop the butter into the flour. Toss around with a large metal spoon until the dough is lightly coated in the flour. Mix the water and lemon juice and pour into the flour and butter. Use a table knife to cut across the bowl several times, chopping the butter into the flour until the dough comes together. When it forms a loose lump, tip it on to a board and quickly shape into a fat slab.
Roll out the dough on a well-floured surface to a rectangle around 38cm x 20cm. Fold the bottom one-third of the dough two-thirds of the way up the pastry rectangle, then fold the other third over that. Press the edges firmly with the pin, then rotate the pastry a quarter turn. Use the rolling pin to make 3 shallow, depressions horizontally across the pastry - this will help keep the edges straight.
Roll into the same size rectangle and start the process again. Continue five more times: rolling into a rectangle, folding, pressing, turning a quarter-turn and making the depressions. Don’t worry too much if the butter makes its way through occasionally in the early stages; just keep the board and pin very well floured to prevent the pastry from sticking. After the very last folding stage, wrap the slab in clingfilm and chill for 1-2 hours (or overnight). If it is very firm when you come to use it, allow the pastry to warm through for a few minutes before rolling.
While the pastry is resting, you can prepare the pie filling. Preheat the oven to 220C/Gas 7. Melt the butter in a medium-sized non-stick saucepan over a low heat. Gently fry the onion and celery for a couple of minutes before adding the Pernod. Continue cooking the vegetables for a further 8-10 minutes until soft, but not coloured, stirring occasionally.
Sprinkle the flour into the pan and cook for a few seconds before gradually adding the milk, while stirring constantly. When all the milk is added, cook the sauce gently for at least 5 minutes, and preferably a bit longer. Stir continually until smooth and glossy. Remove from the heat and stir in the cream, tarragon and salt and pepper to taste. Cover the surface of the sauce with clingfilm to prevent a skin forming and leave to cool for at least 30 minutes.
Line a small baking tin with a large sheet of foil and brush with a little sunflower oil. Put the cleaned trout into the tin, top to tail, and cover with a second piece of oiled foil. Scrunch the edges together to form a large, flat parcel. Bake the trout for 15 minutes, then leave to stand without opening for 15-20 minutes.
When you are ready to assemble the pie, open the foil to reveal the trout. Strip off the skin - it should come away easily from the barely cooked fish - and lift the fish in large pieces from the spine. Put in a bowl. Flip the fish over and do the exactly the same thing on the other side.
Discard the skin and bones, but tip 2 tbsp of any residual juices into the white sauce. Loosen the sauce with a little extra milk if necessary - it should have a smooth, pouring consistency.
Spoon a little of the sauce into a 1.5lt pie dish and top with a few pieces of fish; continue layering the sauce and fish until both are used.
Roll out the pastry on a floured surface until around 2cm larger than the pie dish. Brush the edge of the dish with beaten egg and carefully place the pastry over the filling. Trim and knock up the edges with a sharp knife. Make a cross-hatch pattern across the top and brush lightly with more egg. Put on a baking tray and bake at 200C/Gas 6 for around 30-35 minutes until the pastry is golden brown and the filling is hot. Check every now and then to make sure the pastry doesn’t burn before the filling is ready, and turn down a few degrees if necessary.