In a saucepan put 3 cucumbers, peeled and thinly sliced, 1/2 cup chopped green onions, 1/2 teaspoon salt and a sprinkling of pepper. Add 1 cup water, bring the liquid to a boil, and simmer it until the cucumbers are soft. Stir in 2 tablespoons flour, mixed to a smooth paste with a little cold water, and 2 cups chicken stock. Stir the soup until it comes to a boil, and simmer it for 10 minutes. Add a pinch of cayenne and more salt if needed and strain the soup through a fine sieve, pressing through as much as the vegetable pulp as possible. Add 1 tablespoon chopped chervil and stir the soup over ice until it is cold. Stir in 1/2 cup heavy cream and chill until ready to serve. Garnish each serving with a thin slice of scored cucumber.