In a blender, combine the chopped cucumber with the yogurt, lemon juice, shallot, garlic, dill, parsley, tarragon and the 1/4 cup of olive oil. Blend until smooth. Season with salt and white pepper, cover and refrigerate for at least 8 hours or overnight. Season the soup again just before serving. Pour the soup into bowls. Garnish with the finely diced cucumber, red onion and a drizzle of olive oil and serve.