Peel the potatoes and cook until soft. After draining, let it cool down. In the meantime, cut the onions into thin rings and caramelize them in a pan, then add a pinch of salt. Cut the potatoes into approx. 1.5 cm thick slices. Heat the oil in a large pan, line the bottom of the pan with the potato slices and fry on one side. When all the potatoes are seared, pound the butter through them well. Mix hot milk, approx. 1 teaspoon nutmeg, pepper and the onions. Now add the truffle oil to the mash - preferably a little less, because the truffle taste should not be in the foreground, season with salt and pepper.