1. Drain the salmon, discard the skin and bones, if desired. Flake the salmon with a fork, cover and set aside
2. Preheat the oven to 190°C / 375°F/ Gas Mark 6. Grease and line a 20 x 30cm (8 x 12 inch) Swiss roll tin with non-stick baking paper. Brush with a little more vegetable oil
3. In a food processor, finely chop the watercress. Tip into a large bowl and mix in the egg yolks and season with salt and pepper. In a separate bowl, whisk the egg whites until stiff, then fold them into watercress mixture using a large metal spoon. Pour into the prepared tin and quickly level the mixture, spreading it out into the corners. Bake for 10 - 12 minutes until set and light golden brown
4. Turn out the set mixture onto a large sheet of greaseproof paper. Carefully peel away the lining paper and allow to cool
5. Mix together the soft cheese and crème fraîche. Spread evenly over the cooled roulade, then sprinkle the cucumber, tomato, salmon flakes and herbs over the surface. Roll up from the short end, then carefully transfer to a serving plate. Cover and chill until ready to serve