Peel and wash the potatoes and cook them in boiling salted water for about 20 minutes. In the meantime clean and halve the mushrooms. Peel onions and cut into short strips. Heat oil in a pan and fry mushrooms and onions for about 5 minutes.
Season with salt and pepper. Heat clarified butter in a pan. Fry the steaks for about 4 minutes on each side. Season with salt and coarse pepper, keep warm and let it rest. Drain the potatoes and let them evaporate.
Add milk and butter and mash coarsely with a potato masher, season to taste with salt and nutmeg. Fold in mushrooms and onions. Cut herb butter into slices. Arrange mashed potatoes with steaks and herb butter slices on plates.
Garnish with parsley leaves.