Wash, peel and finely cube the celery. Mix the water with the 2 T lemon juice and a bit of salt and cook the celery in it until soft. Rinse with cold water and drain. Stir the egg yolk well with the mustard, lemon juice, Worcestershire sauce, vinegar, salt and pepper. Whisk the oil in drop by drop (the more oil added, the thicker the mayonnaise). Stir in the sour cream, season to taste and stir until smooth.
Wash, peel, halve and core the apples and cube them finely like the celery. Mix with the mayonnaise, walnuts and celery cubes and chill. Line the plates with the lettuce leaves and hearts and fill with the mayonnaise salad. Garnish with pistachios and parsley and serve.
This salad can be served as a first course or as an accompaniment to meat.