Preheat oven to 220°C/425°F/GasM7 Have ready a lined baking sheet.
Melt butter and lightly sauté onion and ginger until onion just transparent, season with black pepper and nutmeg and allow to cool. Preheat oven to 220°C(200°C for fan assisted ovens) 425°F/Gas M 7
Roll out pastry to a rectangle approx. 30cm x 30cm / 12" x 12 and cut into 4 equal squares.
Combine salmon with onion mix and watercress leaves, reserving a few for garnish. Divide the salmon mixture between the four squares of pastry, placing it in the middle of each.
Lightly brush the edges of one square with beaten egg, then bring up all four corners to meet in the middle, pressing the sides together to seal. Gently knock up the 'seams' and brush the whole with beaten egg. Repeat with remaining pastry squares.
Place on a baking sheet and bake for 15 - 18 minutes until pastry risen and golden.
This also makes an excellent starter - cut pastry into 8 squares and cut salmon into smaller chunks to allow 'wrapping'.