Rinse the liver in cold water and dry it. Peel the onion and cut it into rings. Rinse the sage leaves and dry them. Season the liver with salt and pepper and dust it lightly with flour.
Immediately fry in hot oil for about 1.5 minutes on each side. Then fry the onion in the frying fat and add the sage leaves. Deglaze with the balsamic vinegar and wine. Place the liver on top of the onions and braise for 5 minutes.
Serve with mashed potatoes.
