Potatoes rinse thoroughly and make with the peel about twenty minutes.
Peel the celery and cut into slices. Cook in a little water with a little salt and vinegar (so that the celery does not turn brown) for about five minutes until al dente.
Remove peel from potatoes, cut into slices and pour hot clear soup over them. Season with salt and pepper and simmer for about fifteen minutes.
Cut spring onion into fine rings. Peel ginger and chop finely.
Beat egg yolks with a whisk. While continuing to whisk, add the oil in a fine stream. Fold in yogurt. Season heartily with pepper, juice of one lemon, salt and sugar.
Add drained ginger, spring onion, celery and salad dressing to the potatoes, mix and leave for an hour. If necessary, season with pepper or salt.
Roast hazelnut flakes in a frying pan without adding fat and sprinkle on top.
Serve with: fried egg