Put cabbage, carrots, celery and parsley into a bowl and mix well.
To make the dressing, blend oil and flour in a saucepan, add 125 ml (4 fl oz/ ½ cup) water, the vinegar, mustard, egg and seasonings. Cook over a very low heat until thickened, stirring all the time. Cool slightly, then pour over the salad and mix well. Cool before serving garnished with celery leaves.