A good knob of butter
1 medium onion, peeled and roughly chopped
1 leek, trimmed, roughly chopped and washed
500g Jerusalem artichokes, peeled and cut into quarters
1.5 litres vegetable stock
salt and freshly ground white pepper
2 tablespoons of double cream
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 strain through a fine sieve into a pan. Stir in as much of the reserved stock as you need to get the right texture.
Stir in the cream, check the seasoning and serve.