Cucumber consists mostly of water, so it needs gentle and brief cooking to help it blend smoothly and bring out the flavours and to help prevent it separating once it's blended and chilled.
You can buy Braden Rost or "hot smoked" salmon from good delis or fishmongers, or you could just use poached salmon instead.
1 leek, roughly chopped and washed
1tbsp olive oil
2 cucumbers, roughly chopped (reserve a quarter of one for garnish)
600ml vegetable stock
Salt and freshly ground black pepper
For the garnish
The quarter cucumber, halved lengthways, seeds scooped out and sliced on the angle
80-100g Braden Rost, broken into chunks
A few sprigs of fennel or dill
A spoonful of olive oil
Gently cook the leek in the olive oil, in a covered pan, for 2-3 minutes until soft. Add the vegetable stock, bring to the boil and simmer for 5 minutes, then add the cucumber and remove from the heat. Blend in a liquidiser until very smooth. Strain through a medium-meshed sieve (a fine mesh will trap too many bits) into a bowl set over some iced water to cool it quickly and prevent discolouring. Season to taste. Meanwhile, blanch the slices of cucumber in a little boiling, salted water for 30 seconds and drain and refresh under cold water. To serve, mix the cucumber and salmon together, pour the soup into large chilled soup bowls about 2-3cm deep and arrange the salmon and cucumber in the centre. Scatter the dill over and spoon a little olive oil around.