step 1
Heat the butter in a large pan and add the onions and leeks. Cook for 10 minutes until softened. Add the potatoes and cook for a further minute.
step 2
Add the vegetable stock, season, and bring to the boil. Simmer until the vegetables are tender, about 30 minutes.
step 3
Put into a blender until smooth. Stir in the cream or crème fraîche. Heat through gently and serve.