Peel and cut the potato into 1cm chunks
Trim the leeks, slice in half along the length and clean, then slice into roughly 2cm pieces. Rinse thoroughly
Peel and dice the onion finely
Melt the butter in a large heavy-based pan and add the onion. Soften over a low to medium heat for about 5 minutes, stirring constantly.
Add the leeks and soften for a further 5 minutes
Add the potato and stock. Season with a little salt and pepper
Bring the mixture to simmering point and cook for 25 minutes until the potato is soft and the leeks are cooked through
At this stage, you can blitz the mixture to make a smooth soup if you like.
Adjust the seasoning and serve with a dollop of creme fraiche