Peel, wash and cut the celery tubers into 4 thick slices. Pre-cook in boiling salted water for about 5 minutes. Drain and dab dry with kitchen paper. Whisk the egg.
Turn the celery slices in the egg first. Carefully spread some mustard on one side and then turn the slice in the breadcrumbs. Heat the fat in a coated pan and fry the vegetable slices in it until golden brown.
Season yoghurt with salt and pepper. Wash chives, dab dry, cut into small rolls and stir in. Arrange celery slices with the chive sauce on plates. Serve with a mixed salad in a light vinegar-oil vinaigrette.