Heat the butter in a large saucepan, then gently fry the onion, potato and leeks for 5 minutes until they begin to soften.
Add the parsley, stock, bay leaf and seasoning. Bring to the boil and simmer for gently 25 minutes.
Whizz the soup together in a liquidizer until smooth.
Nice served with a dollop of fresh cream or creme fraiche and a chopped parsley garnish.