Peel the potatoes, cut them into smaller pieces and rinse them in plenty of cold water.
Clean the leeks and slice the white and light green part.
Melt the butter in a large saucepan and sauté the onion and leeks for a couple of minutes.
Then add the potatoes, pour in the stock and bring to the boil. Turn the heat down, put the lid on and let the soup simmer for 20 minutes, or until the vegetables are tender.
Blend the soup until smooth, add the cream and season with salt and plenty of freshly ground pepper.Adjust the consistency with more stock if you would like the soup thinner.
Serve with a little crème fraîche on top, a good olive oil and finely chopped chives.