Boil the potatoes in their skins in advance and let them cool, then peel them. Peel the onion. Wash the mushrooms.
Finely chop the onion, fry in vegetable oil until golden brown. Add mushrooms.
Slice the potatoes and add to the contents of the frying pan. Mix everything thoroughly. Fry over medium heat for 3-4 minutes.
Add butter without sparing, turn off the stove and close the pan with a lid.