Trim and remove strings from the celery and cut it into 3" pieces. Heat a stainless steel kettle and melt the butter. Add the onions and cook, stirring, for 10 minutes or until the onions are golden. Add the celery, apples, tomatoes, sherry, salt and pepper to taste and sweat the mixture, covered with a buttered round of wax paper and the lid, over low heat, stirring occasionally, for 1 hour. Add the chicken stock, bring all to a boil and simmer for 10 minutes. In a food processor fitted with a steel blade puree the mixture in batches and strain the puree through a course sieve into a bowl.

To serve the soup hot transfer it to a kettle, bring to a simmer and ladle it into heated bowls. Garnish with chives.

To serve the soup cold, chill it, covered, for 2 hours, ladle it into bowls and garnish it with salted whipped cream and a sprinkling of chives.

This makes about 6 cups, serving 6 to 8.