Wash the rocket salad and grapes. Halve the grapes and place in a bowl with the salad. Peel the celery stalks and cut them diagonally into fine slices. Add this to the salad. Cut the apple into slices. Briefly roast the walnuts in a pan without adding any fat and add the parsley as well.
For the dressing, put the mustard and vinegar in a shaker. Add three times the amount of olive oil. Add the yoghurt and pepper and shake well. Add the roasted walnuts and apple to the salad. Then add the dressing and stir gently. Finally, sprinkle the blue cheese in flakes over the salad.