Peel the onions and cut into very small cubes. Cut the leek lengthways into thin strips, wash and drain. Peel the parsnips, carrots and celery and cut into thin strips. Cut the parsley very finely.
Heat a high-sided pan with the butter. Cover the vegetables with the exception of the leek and parsley and stir them occasionally so that nothing burns. After about 10 minutes add the leek and parsley. Continue to steam until everything is done, but still a little firm to the bite.