To make the pastry: sieve the flour and salt into a large bowl. Roughly break the butter in small chunks, add them to the bowl and rub them in loosely. You need to see bits of butter.
Make a well in the bowl and pour in about two-thirds of the cold water, mixing until you have a firm rough dough adding extra water if needed. Cover with cling film and leave to rest for 20 minutes in the fridge.
Turn out onto a lightly floured board, knead gently and form into a smooth rectangle. Roll the dough in one direction only, until 3 times the width, about 10cm x 25cm. Keep edges straight and even. Don't overwork the butter streaks; you should have a marbled effect.
Fold the top third down to the centre, then the bottom third up and over that. Give the dough a quarter turn (to the left or right) and roll out again to three times the length. Fold as before, cover with cling film and chill for at least 20 mins before rolling to use.
To make the filling: bake the salmon in a preheated 180 C / Gas 4 oven for 15 minutes or until just cooked. Flake the salmon into chunks and sprinkle over base of a pie dish
Scatter cooked broccoli and mushrooms over the salmon. Season with thyme, dried mixed herbs, salt and pepper.
To make the sauce: melt butter in pan and add plain flour and stir for 1-2 mins on the heat. Take pan off heat and gradually whisk in 300ml of milk.
Return to heat and simmer for 8-10 minutes stirring continuously. Season with salt and pepper
Pour the sauce over salmon and broccoli mixture. Roll out pastry and place on top. Decorate the pie with the leftover pastry. Brush with beaten egg yolk, if desired.
Bake at 180 C / Gas 4 for 30-40 minutes or until pastry is golden brown.