Celeriac and apple soup

1 onion, chopped
850 ml (1 1/2 pints) vegetable stock
1 teaspoon curry powder or paste (optional)
250 g (9 oz) celeriac (about half a standard size), cut into 1cm (1/2 inch) cubes
1 large cooking apple, peeled, cored and cut into quarters
chopped, fresh parsley, to garnish

1. Put the onion in a large pan, with enough stock or water to cover and the curry powder or paste, if used. Cover the pan and simmer for 5 minutes.
2. Add the celeriac and the remaining stock and cook, covered, for a further 15 minutes.
3. Add the apple and cook for a further 5 minutes, until the apple has collapse.
4. Either mash the soup with a potato masher or liquidise it in a food processor or liquidiser. Serve sprinkled with parsley.