Cut the boiled potatoes into slices. Peel onions and cut into slices. Fry potatoes in hot clarified butter for about 10 minutes until golden brown. Fry onions and chopped hazelnuts for about 5 minutes. Season with salt, pepper and grated zest of 1 organic lemon.
Wash and cover medium-sized waxy potatoes and cook for about 20 minutes. Drain, quench, peel and let it cool down, preferably overnight. Do not cut the potatoes too small and fry them in a large pan in hot clarified butter or oil, first from one side until golden brown. Then turn and fry until done.
To make the potatoes nice and crispy, do not stir. Just turn them from time to time, so they don't break.