For the mayonnaise, stir together egg yolk with 1 tablespoon lemon juice in a mixing bowl, then add the oil first by drops, then pour in a thin stream while stirring until a creamy mayonnaise results. Stir and season with salt and pepper. Rinse and peel the celery, put aside some green for garnish. Cut celery diagonally into thin slices. Coarsely chop the walnuts.
Rinse the apples, cut into quarters, remove the cores and cut into 2-3 cm (approximately 1 inch) pieces, immediately drizzle with the remaining lemon juice. Mix the salad ingredients with the mayonnaise and season again heavily. Serve garnished with celery leaves.