Method

1. Melt the butter in a medium saucepan over medium heat. Add the celery and cook, stirring occasionally, for 5 minutes or until starting to soften. Add the flour and stir with a balloon whisk for about 1 minute until the mixture is bubbling and leaves the sides of the pan.
2. Remove the pan from the heat and gradually stir in half the milk, stirring constantly, until smooth. Gradually add the remaining milk and cream, stirring until smooth. Return the pan to medium heat and stir constantly until the sauce thickens and starts to simmer. Reduce the heat to low and simmer, stirring frequently, for 2 minutes.
3. Transfer the sauce immediately to a large heatproof bowl and stir in the cheddar, spring onions, capers, dill and mustard. Taste and season well with salt and pepper.
4. Place in the fridge for about 1 hour or until cooled to room temperature.
5. Preheat the oven to 200˚C/400°F (180°C/350°F fan-forced). You will need a 1.5 litre (6 cup/2pt 10fl oz) capacity ovenproof dish to cook the pie.
6. When the sauce has cooled, add the salmon and stir gently to combine evenly.
7. Use a lightly floured rolling pin to roll out the pastry on a lightly floured surface to the shape of the dish until about 4mm/⅛in thick. Spoon the cooled salmon mixture into the dish, then brush the edge of the dish with a little of the egg wash. Place the pastry over the filling to cover, then use a small sharp knife to run around the edge of the pie dish on a slight angle to trim the excess pastry. Use a fork to press around the edge of the dish to seal. Cut a cross in the top of the pie and then brush the pastry with a little egg wash to glaze. Decorate the top with the off-cuts of pastry, if desired, and then glaze again with the little of the egg wash.
8. Place the pie on a baking tray and bake for 45 minutes or until the pastry is dark golden, crisp and cooked through. Serve straight from the oven.