Trim the leeks and cut into slices. Peel the onion and chop. Peel the potato and cut into small chunks. You don’t need to worry too much about cutting the vegetables evenly as the soup will be puréed.
Heat the butter and oil in a pan, add the leeks, onion and potato and two sprigs of parsley and heat gently for about 5 minutes until starting to soften.
Add the water and simmer gently for 15 minutes or until the potato is cooked.
Purée until smooth, then pour the soup back into the pan and heat through.
Finely chop the remaining parsley and add to the soup.