The day before, cook the potatoes in their skins, then drain, rinse and peel. The next day grate finely on a grater or press through a potato ricer. Then add salt, pepper, onion, seasoning and hot vinegar mixture, mix well with potatoes and season again. Then add the oil and fold it in. Pile up the finished lettuce like a hill, smooth it and cut strips with the back of a knife. Let it stand for 1 hour and garnish with a parsley leaf on the top before serving. Or simply top with hard-boiled egg slices and place a parsley leaf on top.