Mix the vinegar with the mustard, salt, pepper and sugar, gradually fold in the oil with a fork.
Peel, quarter and core the apples. Cut the apple quarters into thin wedges and mix immediately with the dressing so that they stay nice and light. Remove the thick stalks from the rocket, wash the remaining leaves and spin dry.
Mix the rocket with the apples and dressing and distribute on plates. Roughly chop the walnuts and sprinkle over them.
Slice the cheese into shavings and pour over the salad.