Cook onions, 2 cucumbers, and leeks in butter until soft but not brown, about 30 minutes. Add chicken broth, salt, and pepper and bring to a boil. Make a roux with 1 tablespoon butter and flour; whisk into the soup. Simmer gently for 30 minutes to develop the flavor. Remove from heat and whirl in a blender until smooth. Strain puree into a bowl and add cream, dill weed, lemon juice, hot-pepper sauce, and chopped cucumber. Chill thoroughly. Taste to correct seasonings. Serve in soup bowls.