Peel the potatoes, cut into quarters and boil in salted water until soft. Strain, let steam out (possibly in a slightly preheated oven).
Mash with a potato masher, press through a potato press or strain through a sieve. Add the butter and let it melt, stir in.
Heat the milk, gradually folding it in to make a thick, smooth paste. Season with salt and grated nutmeg.
For the roasted onion, peel the onion, cut into slices. Heat the oil in a coated pan, roast the rings in it and place them on top of the mashed potatoes.
If you increase the amount of butter, your figure will not thank you, but your taste buds will. You can also refine mashed potatoes well with a dollop of whipped cream, and some add a touch of truffle oil.