Cut the celery and the leek (only up to the light green) into fine strips, and the apples and cut into thin slices. Mix the mustard, pepper and salt and maple syrup, beat with the oil until it emulsifies and mix everything with the vegetables.
Coarsely chop the walnuts and lightly toast them in a pan without fat. Spread the cheese in pieces over the salad, sprinkle the chervil, roughly chopped, and finally sprinkle the walnuts loosely over the top.