Cucumber and Yogurt Soup with Walnuts

1 cucumber
4 garlic cloves
1/2 teaspoon salt
3/4 cup walnuts
1 1/2 ounces day-old bread, torn into pieces
2 tablespoons walnut or sunflower oil
1 2/3 cups plain yogurt
1/2 cup cold water or chilled still mineral water
1 to 2 teaspoons lemon juice
For the garnish:
scant 1/2 cup coarsely chopped walnuts
1 1/2 tablespoons olive oil
sprigs of fresh dill

Cut the cucumber in half and peel one half of it. Dice the cucumber flesh; set aside.
Using a large mortar and pestle, crush together the garlic and salt. Add the walnuts and bread and continue working together.
When the mixture is smooth, slowly add the walnut or sunflower oil and combine well.
Transfer the mixture into a large bowl and beat in the yogurt and diced cucumber. Add the cold water or mineral water and lemon juice to taste.
Pour the soup into chilled soup bowls to serve. Garnish with the chopped walnuts, drizzle with the olive oil, and arrange the sprigs of dill on top. Serve at once.
Serves 5-6.