Make the dressing up to a day ahead, and simply stir before using. Refrigerate the separated leaves in a plastic bag up to 4 hours ahead.
Separate the chicory leaves and put them in a serving dish. Mix together the oil, vinegar, mustard and sugar with salt and pepper to taste. Pour over the leaves and toss with a spoon and fork so that all the leaves get coated. Serve straight away, before the dressing has a chance to discolour and soften the leaves.