Japanese Artichoke and Rocket Soup

• 700 g (1 lb 9 oz) Japanese artichokes, trimmed
• 25

Japanese artichokes, or crosnes, are hard to find except from specialist greengrocers. If you cannot obtain them, simply replace with Jerusalem artichokes.

Unless the Japanese artichokes are very young, blanch them in a large pan of boiling water for 10-15 seconds, drain well, then put them in a tea towel with a little coarse salt and rub gently to remove the skins.

Heat the but