Preheat the oven to 160°C. Line a baking tray with non-stick baking paper.
Place the butter, salt and water in a saucepan and bring to the boil. Remove from the heat and stir in the flour. Return the saucepan to a medium heat and stir briskly until the mixture forms a soft dough and leaves the side of the saucepan dry. Transfer to a bowl without cooling the mixture, then add the eggs one at a time, beating thoroughly after each addition. The dough should be smooth and glossy.
Fill a piping bag fitted with a plain 10 – 15mm nozzle. Pipe eclairs about 4-5cm strips on to the prepared baking tray. Brush the top of each éclair with a very light coating of egg wash. Place into a preheated oven and bake for 20–25 minutes, until golden brown outside and dry inside. Transfer to a wire rack to cool.
In the meantime, prepare the salmon mousse. Process the smoked salmon and cream cheese in a food processor until combined. Add the zest of half a lemon and check the seasoning. Transfer in a piping bag.
Combine the rest of the lemon zest with some crème fraiche for the icing.
when ready to serve, fill the eclair with the smoked salmon mixture. Dip the top of the éclair in the crème fraiche mixture and dip in the chive.
Set aside on the serving platter with extra slices of smoked salmon and serve within an hour or so to ensure the eclair keeps its freshness.