Grate the cucumber, reserving 3 cm to slice for garnishing.
Oil a mould and chill in the refrigerator.
Blend together the cucumber, cream cheese and sour cream in a food processor and season with salt, freshly ground black pepper and nutmeg.
Stir in the lemon juice.
Pour the stock into a small saucepan and soak the gelatine in it.
When spongy, warm the liquid very gently until it is clear.
Add the liquid to the cucumber mixture, mixing thoroughly.
Pour into the mould and chill in the refrigerator until set.
Place a plate over the mould and invert the plate and mould together.
With a sharp shake, dislodge the mousse onto the plate and decorate with cucumber slices, rocket and tomatoes.
This mousse should be eaten within 24 hours.
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