Chilled Cream of Cucumber Soup

• 3 medium cucumbers, about 1½ lb., peeled and chopped
• 1 medium onion, finely chopped
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Put the cucumbers, onion and stock into a heavy saucepan, cover and simmer gently for 15 minutes. Cool a little, then pour into an electric blender and reduce to a purée. Strain and return to the saucepan. Season to taste with salt and pepper. Mix the arrowroot with 1 tablespoon of water and stir in