heat one tablespoon of the olive oil and all the butter in a heavy based frying pan

fry the onions over a low heat for 3 mins 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 calfs liver with salt and freshly ground black pepper and fry for 30 secs (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, creme de cassis and parsley

place the liver and onions onto hot plates and serve at once