Melt the butter in a saucepan and gently cook the onion and leek until soft. Add the Jerusalem artichokes and the vegetable stock and season lightly with salt and pepper. Bring to the boil, lower the heat and simmer for about 25 minutes until the artichokes are tender. Strain about a quarter of the stock into a jug and set aside.
Whiz the rest of the soup in a blender until smooth, then