Bring the water, butter and salt to the boil, reduce the heat, add all of the flour at once, stir with a risotto spoon until a smooth dough is formed which separates from the base of the pan. Remove the pan from the heat, allow the mixture to cool a little, add the mustard. Using the dough hook on a hand mixer, gradually mix in the egg; the dough should be soft but not runny. Using two teaspoons, shape the dough into approx. 16 mounds and transfer to a baking tray lined with baking paper, chill for approx. 15 mins.
Approx. 30 mins. in the centre of an oven preheated to 180°C. Do not open the oven during baking! Then switch the oven off and leave to dry with the door ajar for approx. 30 mins., remove from the oven.
Cut open the puffs with scissors and leave to cool on a rack.
Puree the salmon and cream.
Stir the horseradish mousse and dill into the salmon mixture, season with salt, cover and refrigerate for approx. 1 hr. Transfer the mixture to a piping bag with a round nozzle (approx. 9 mm in diameter). Fill the mustard puffs with the salmon mousse.