There are various versions of this Jerusalem artichoke soup. I like Auguste Escoffier's version which included hazelnuts. Not many farmers grow Jerusalem artichokes these days. Perhaps they have gone out of vogue, or maybe punters are put off because of their repetitive reputation. Some recipes contain bacon, which is cooked with the onions at the beginning, but it's up to you.
1 onion, peeled and roughly chopped
1kg Jerusalem artichokes, peeled and halved
1 litre vegetable stock
10-15 shelled hazelnuts, lightly toasted
Salt and freshly ground white pepper
1tbsp chopped parsley
2-3 slices of bread, 1cm thick, crusts removed
In a large saucepan melt half the butter and gently cook the onions and Jerusalem artichokes with a lid on for 4-5 minutes, stirring every so often. Add the stock and milk, season, bring to the boil and simmer for about 30 minutes until the artichokes are tender. Add the hazelnuts and blend until smooth in a liquidiser, then strain through a sieve or keep as it is. You can adjust the consistency with more milk or stock if it's too thick. Meanwhile cut the bread into rough 1cm cubes and fry in the butter until golden, transfer to some kitchen paper and season. Simmer the parsley in the soup for a minute or so and serve with the croutons.
To see Mark Hix's exclusive cookery videos, see http://www.independent.co.uk/hixcooks