Heat the oven to 200c/fan 180c/gas 6. Cut a pack of ready-rolled puff pastry into quarters, put on a baking sheet and prick all over with a fork.
Mark a 1cm border with a knife and brush with beaten egg. Bake for 10 minutes until golden. mix 6 tbsp crème fraîche with 2 tbsp wholegrain mustard and season.
Spread half over the baked pastry case middle and cook for another 5 minutes. Grill or pan-fry 2 lightly smoked salmon portions for 4 minutes, or until cooked through. Flake over the pastry squares and scatter with half a bag of watercress. Mix the remaining crème fraîche with 1 tbsp cider vinegar and drizzle to serve.