Peel, wash and chop the potatoes. Cook in boiling salted water for about 20 minutes until soft. Peel onions and cut into thin rings. Cut bacon into small cubes. Heat oil in a pan. Add onions and bacon and stir in over medium heat for about 10 minutes.
Drain the potatoes. Add 50 g butter, buttermilk and milk and mash with a potato masher until mashed. Season to taste with salt and nutmeg. Add 25 g butter to the onion-bacon mix and melt. Arrange mashed potatoes on plates and spread onion-bacon-melt on top