Ingredients (to serve 4):
Place the onion and garlic in a large saucepan and add 150ml of the stock. Bring to the boil, cover and simmer for 5 mins.
Add the potato, and all but a few shreds of the leek. Pour in the remaining stock, add the dried herbs and season lightly. Bring to the boil, cover and simmer for 25 minutes until tender.
Transfer the mixture to a food processor or blender. Blend until smooth and return to the saucepan.
Blend in Llaeth y Llan natural yogurt and reheat gently without boiling. Adjust the seasoning if necessary.
Ladle into bowls and top with the reserved shredded leek and chopped chives.