Heat the oil, peel the onions, cut into rings and sauté until translucent. Deglaze with wine and stew for 20 minutes until the wine has evaporated. Remove the onions and set aside.
Melt the butter, cut the liver into narrow strips, add to the hot butter and cook for 4 minutes, stirring constantly. Add the onions, let them get hot and season with salt and pepper.