Peel off half the cucumber skin, then roughly chop the cucumber. Put it on to a plate and sprinkle with a little salt. Set aside for 20 minutes.
Rinse the cucumber with cold water and drain well in a sieve. Put the walnuts, garlic, dill, bread and oil into a blender or food processor and whiz until finely chopped. Add the cucumber and yogurt and blend again until the cucumber is finely chopped. Mix in the water, lemon juice and season with salt and pepper to taste. Chill well.
Ladle into glasses. Drizzle the top with a little extra olive oil, sprinkle on a few walnuts and a sprig or two of dill. Serve with strips of toasted pitta bread, if liked.