Preheat oven to gas 7, 220°C, fann200°C.
Lightly dust the work surface with flour and roll out the pastry to a rectangle about 28x40cm (12x16in). Cut in half lengthways.
Cut the salmon fillets in half lengthways and place down one side of each pastry strip. Carefully remove rocket, watercress or spinach leaves from stems and discard stems. Top the salmon with the leaves.
Brush one edge of the pastry with egg white and fold the pastry over, very tightly, to enclose the salmon. Press down well.
Place on a baking tray. Brush with egg white and sprinkle with the Parmesan and, if using, fennel seeds.
Bake for 15-20 minutes until the rolls are golden brown and puffed up. Allow to cool for 5 minutes before cutting into slices. Delicious hot or cold.
Serve these salmon rolls as canapés, or cut into larger chunks to make a great main course.