1. Preheat the oven to 400°F (200°C). Lightly oil a large baking sheet. Mix the cream cheese and watercress together until combined; season with salt and pepper.

2. Cut the salmon fillet vertically in half. Place the pastry sheet on a lightly floured work surface and roll out into a 12 × 10in (30 × 25cm) rectangle. With a sharp knife or kitchen scissors, trim the edges straight. Pierce all over with a fork. Transfer to the baking sheet.

3. Place 1 salmon fillet in the center of the pastry. Spread with the cream cheese mixture, then top with the other salmon fillet. Brush the edges of the pastry with water. Fold the long sides up and overlap over the salmon, and press the seam closed. Fold up the shorter sides and press closed. Brush with beaten egg and decorate with the trimmings, if desired, and brush again. Pierce two or three holes in the pastry.

4. Bake for 30 minutes, until the pastry is golden brown.

5. Let stand for 3 minutes, then slice and serve hot.

Good with hollandaise sauce or lemon wedges.