Preheat the oven to 200°C / 400°F / Gas mark 6. Spread walnuts out on a baking tray and put in hot oven for about 4 minutes until lightly roasted. Remove from oven and cool.
To make the dressing: Drain the silken tofu by putting it in a bowl lined with kitchen paper and patting dry. Transfer French dressing to a small liquidizer together with the silen tofu and the caster sugar. Blend for around 30 seconds until creamy. Put into a bowl ready to serve.
Continue with the salad: Cut the cucumber in half lengthways, and then thinly slice to make half-moon slices. Mix these with the fennel and watercress.
Arrange whole green lettuce leaves on four plates, and pile the cucumber mixture on top. Scatter with the blackberries, raspberries and roasted walnuts and serve, passing the dressing around separately.