Preheat the oven to 220C/200C Fan/Gas 7. Line a baking tray with silicone or baking paper.
Pour 125ml/4½fl oz water into a pan and add the butter and salt. Bring to the boil slowly, then boil for 1 minute. Remove the pan from the heat, pour in the flour all in one go and beat continuously for a few minutes, until the mixture is smooth and well combined and comes away from the sides of the pan cleanly – you should be able to hear the mixture making a popping noise in the pan.
Turn the choux dough out into the bowl of a food mixer and allow to cool for 2-3 minutes. Once cooled slightly, beat in the eggs, one at a time, then continue to beat for 2-3 minutes, or until the mixture is smooth and glossy (you could also do this using a large bowl and an electric whisk).
Spoon the choux dough into a piping bag fitted with a large, plain nozzle, then pipe blobs 1.5cm/½in in diameter onto the prepared baking tray, leaving a 3cm/1¼in gap between each. Smooth the tops of the balls slightly with a wet fingertip, then sprinkle 1 tablespoon of water over the balls on the tray.
Bake in the oven for 12-15 minutes, or until risen and golden-brown, then remove from the oven and set aside to cool.
Meanwhile, for the filling, blend the smoked salmon in a food processor until smooth. Add the cream cheese and 150g/5½oz of the crème fraîche and blend again until very smooth. Season, to taste, with plenty of freshly ground black pepper and a squeeze of lemon juice and blitz once more.
Spoon the mixture into a clean piping bag and chill in the fridge until ready to use.
Whisk the remaining crème fraîche with the remaining lemon juice. Season well, transfer to a small serving bowl and set aside. Place the chopped chives in a bowl.
For the watermelon and feta bites, stack the watermelon and feta cubes on the serving plates, garnising with a basil leaf. Drizzle with a little sherry vinegar.
When the choux buns are cool, pierce a hole in the top of each and pipe the smoked salmon mixture into the middle.
Dip the tops of the filled choux buns into the seasoned crème fraîche, then dip into the chopped chives. Divide among the serving plates, or serve the choux buns and watermelon and feta bites on separate platters as canapés.