Jerusalem artichoke soup

• 2 lb (900 g) Jerusalem artichokes
• 1 medium-sized onion

Peel the artichokes, placing each one when peeled in cold water with a little vinegar or lemon juice to prevent discoloration, then cut them up. Peel and finely chop the onion.

Melt the butter in a large saucepan over low heat. Add the onion and fry gently for 5 minutes to soften but not brown. Add the artichokes and toss them in the hot butter. Stir in the stock and season with salt an