Heat the olive oil in a large pan. Add the shallots and celery and cook gently, stirring occasionally, until the vegetables soften. Stir in the tomatoes and sugar and cook for a few minutes before adding the stock. Bring to the boil and simmer gently, covered, for 20 minutes.
Cool slightly, then sieve through the medium disc of a food mill. Alternatively the soup can be pureed in a blen