keep warm.
Peel the potatoes, cut them into small pieces, and boil them in salted water for about 20 minutes until tender. Deglaze the pan with the broth, stir in the mustard, and simmer the sauce for 2–3 minutes. Finally, stir in the butter and, if desired, a splash of apple cider vinegar.
Drain the potatoes and let them steam briefly. Add warm milk and butter, mash until smooth, and season with salt and nutmeg.
Serve the liver, golden-brown onions, and mashed potatoes with the sauce. Enjoy!