Heat one tablespoon of the olive oil and all of the butter in a heavy-based frying pan. Fry the onions over a low heat for 30 minutes, or until soft. Season lightly with salt and freshly ground black pepper, place into a bowl and set aside.
Heat the remaining olive oil in the frying pan until hot. Season the calf’s liver with salt and freshly ground black pepper and fry for 30 seconds (you should keep it pink inside). Remove and add to the onions in the bowl and mix together.
Place the liver and onions back into the frying pan. Quickly heat through and add the vinegar, crème de cassis and parsley.
Place the liver and onions onto hot plates and serve at once.