Wash the liver, trim away the membranes and large veins and wash again with cold water. Dry it and cut it into thin slices or strips.
Fry or saute in the hot butter and arrange in a platter. In the same butter, saute the chopped onion until golden, place atop the liver, sprinkle with salt, black pepper and parsley.
You can serve the dish topped with tomato, mushroom or cream sauce.