1. Melt the butter in a wide saucepan and add the onion and garlic. Gently stir and cook for 6-8 minutes or until softened, but not coloured. Add all but 3 tbsp of the cucumber, stir and cook gently for another 4-5 minutes, until fragrant.
2. Pour in the stock and bring to the boil. Reduce the heat and simmer for 5-6 minutes. Remove from the heat and allow to cool for 10-15 minutes.
3. Transfer to a blender with the chopped herbs and blend until very smooth. Set aside to cool completely, then add the yogurt, season and blend again until smooth. Chill for at least 2 hours or ideally overnight.
4. When ready to serve, ladle the chilled soup into shallow bowls. Scatter the crayfish tails and the reserved diced cucumber over the top. Drizzle over the lemon and chilli oils before serving the soup, garnished with a few extra sprigs of dill.