Rinse potatoes and make in water with closed lid 20-25 min.
Clean the lettuce, rinse thoroughly and shake dry. Coarsely crumble feta. Coarsely chop walnuts.
Peel and finely dice onion. Bring clear soup and onion to boil. Season with salt and pepper and simmer on low heat for about 2 min.
Drain potatoes, rinse, remove peel and cut into thick slices while still hot. Pour hot onion broth over them and simmer for about half an hour.
Stir pepper, vinegar, salt and sugar, add walnut oil. Pour over the potatoes form, mix carefully, season.
Fold in lettuce and feta. Sprinkle with walnuts.