Mix the egg yolk and mustard in a high container with the whisk of the hand mixer. Slowly beat the oil into the mixture. Mix with yoghurt. Season to taste with salt and pepper. Wash and peel the celeriac, celery and carrots and cut them into fine strips. Drain the mandarins.
Chop walnuts coarsely. Wash chives, dab dry and cut into fine rolls. Layer all ingredients in glasses. Toast bread, cut out angels and place on the salads. Garnish with salad and celery green if desired