Put the walnuts in a plastic bag with the icing sugar and 1 tablespoon water and shake them until coated. Arrange the nuts on a baking sheet and roast in a preheated oven, 180°C (350°F), Gas Mark 4, for 5 minutes or until gold and crusted.
Separate the chicory leaves and put them into a large salad bowl with the rocket and radicchio. Crumble over the cheese and add the walnuts. Toss carefully.
Make the dressing by whisking together the mustard, vinegar and oil. Drizzle the dressing over the salad, mix lightly and serve.