Cook the potatoes in salted water (it is best to use enough salt so that they no longer need to be seasoned) until cooked for 20 minutes, drain, evaporate and place in a bowl.
Put 50 g butter in the hot saucepan to melt. Put the potatoes through a press and stir in enough milk with a whisk until you get a nice puree. Season to taste with nutmeg and possibly also salt.
Heat the remaining 20 g butter in a pan. Brown the onion rings or breadcrumbs in them and serve the mashed potatoes sprinkled with them.