Melt the butter in a large frying pan over a high heat. Add the garlic, cook for a minute or two and then add in the spinach and watercress. Stir until the leaves just wilt. Press the spinach and watercress through a sieve firmly, pressing as much moisture from them both as possible. Roughly chop, discarding any tough stems, then put in a mixing bowl with the dill and lemon zest. Stir well. Season with salt and pepper.
Dry the salmon fillets thoroughly on kitchen paper and season them generously. And lay on top of the puff pastry, leaving enough pastry to the side to roll over and seal on the opposite side.
Stir the horseradish and the crème fraiche into the spinach and watercress mix, then spread evenly over the fillets.
Top with decorations made from the pastry trimmings if desired and then brush again with beaten egg. Chill for at least 20 minutes. Heat the oven to 200°C/fan180°C
Once chilled, make a few small holes in the top of the pastry for the steam to escape, brush with the remaining beaten egg and bake for 25-30 minutes until puffed and golden and a few juices leach out of the holes. Rest for a few minutes before slicing, or leave to cool a little and serve warm.