Wash and peel the potatoes and cook them in boiling salted water for about 20 minutes. Peel onions and cut into rings. Bring milk and 15 g fat to the boil. Drain potatoes and mash them with a potato masher.
Add milk and stir in. Season to taste with salt and nutmeg. Melt the remaining fat in a pan and fry the onion rings in it until golden brown. Fill mashed potatoes into a bowl, pour onion rings and fat over it.