Mix the egg yolks and 1 teaspoon lemon juice vigorously in a tall beaker with the whisks of the hand mixer. Beat the oil first drop by drop, then in a thin stream. Whip the cream slightly (e.g. milk frother) and fold into the mayonnaise. Season to taste with salt and pepper.
Coarsely chop the walnuts, except for 4 halves. Peel and wash the celery and put some celery green aside. Wash apples, quarter them and remove seeds.
Cut both into fine pencils. Sprinkle with 2 tbsp. lemon juice. Mix celery, apples, chopped walnuts and mayonnaise. Season to taste with salt, pepper and sugar. Garnish with walnut halves, apple wedges and celery green.