La Cucina Veneziana by Guiseppe Maffioli dates this dish back to the Romans, then goes on to say that there are so many versions that he will give all of them - most are minor variations on the central theme of liver and onions with one uniting thread: use the same weight of onions as liver. He also recommends soaking the liver slices in cold water with a few drops of vinegar, or alternatively, milk, for about an hour before cooking.
4 T butter
4 T vegetable oil
1 lb onions, thinly sliced
1 lb calves liver, thinly sliced
salt and pepper to taste
1 t fresh Italian parsley, chopped
Fresh lemon quarters
milk or a few drops of vinegar if you decide to soak the liver
Heavy frying pan.
IN a heavy frying pan, heat the butter and oil and add the onions. Cook the onions until just golden, stirring occasionally.
WHEN the onions are soft, add the slices of liver and fry quickly - the time depends on how thick the slices are and your personal taste - well cooked thick slices can take up to two minutes on a side.
SEASON with salt and pepper, sprinkle with parsley, and serve on a heated serving platter, with lemon quarters for those who like a squeeze of lemon (one recipe calls for squeezing on some lemon just as the cooking is finished, before adding salt, pepper and parsley).
Maffioli recommends serving the dish with polenta, as it would be served in Venice.