Rinse celery, pluck off the green leaves and chop and dice the rods. Peel and finely grate the celery root. Mix the celery root gratings with the lemon juice. Rinse the apples, dry, quarter, core and dice. Mix together the celery, celery root and apples in a bowl.
Mix the creme fraiche with the milk until smooth and season with pepper. Place half of the nuts with the cream mixture in the bowl and mix well. Let soak for at least 1 hour in the refrigerator.
Cut pineapple into small pieces. Gently stir into the salad.
Arrange the salad in bowls and sprinkle with the remaining walnuts.