Gently soften the leeks in the melted butter for 3-4 minutes
Add the potato and celeriac, then pour over the stock and bring to the boil
Simmer gently for 15 minutes before blending with a stick blender or in batches in a liquidiser. If you prefer a chunkier soup, just use a slotted spoon to remove 2 spoonfuls of the vegetable mix before blending.
Check and add salt and pepper as required (this will depend a lot on the stock you use, so it is worth leaving it till the soup is cooked.
Serve with a teaspoon of low fat creme fraiche and/or fresh parsley chopped over