Combine egg yolk with 1 tablespoon of lemon juice in a mixing bowl and gradually add oil, stirring constantly, until sauce becomes creamy. Add cream, mix well and season with salt and pepper.
Rinse and peel celery and set aside green parts for garnishing. Cut the rest diagonally into thin slices. Chop walnuts coarsely.
Rinse apples and cut into quarters, core and cut into 2-3 cm (approximately 1 inch) cubes.
Sprinkle apples with remaining lemon juice.
Combine salad ingredients with mayonnaise and season with salt and pepper. Garnish with celery leaves and serve.