Wash the arugula salad and the grapes. Cut the grapes in half and put them in a bowl with the lettuce. Peel the celery stalks and cut them diagonally into fine slices. Add this to the salad. Cut the apple into slices. Briefly toast the walnuts in a pan without adding fat and also add them with the parsley.
For the dressing, put the mustard and vinegar in a shaker. Add three times the amount of olive oil. Add the yogurt and pepper, then shake well. Add the toasted walnuts and apple to the salad. Then add the dressing and mix gently. Finally, sprinkle the blue cheese in flakes over the salad.
