Heat the olive oil and half the butter in a large frying pan over low heat. Add the onions and cook for 5 minutes and then add the wine. Cook for a further 15 minutes, stirring from time to time, until the onions are very soft and golden in colour. Season with salt and pepper and transfer to a plate.
Melt the remaining butter in the frying pan. Increase the heat to medium and fry the liver for 1 minute on each side to brown. Return the onions to the pan and cook for a further 2 minutes or until the liver is browned on the outside but still a little pink on the inside. Remove from the heat.
Add the chopped parsley lemon zest. Serve with lemon wedges – buon appetito.
Note: Use the freshest calf’s liver for this recipe. If you buy the calf’s liver unsliced, make sure to slice it thinly.