Begin by sautéing thinly sliced onion in a combination of butter and olive oil (or just butter) over medium to medium-low heat in a wide skillet until the onion is well-wilted and very soft. The onion should not color—regulating the heat carefully and add a bit of water or wine and salt will help the wilting process along and prevent browning.

When the onion is ready, push it to the edges of the skillet. Add a bit more olive oil if need be, raise the heat, and add sliced calf's liver, trimmed and sliced into thin strips, to the skillet. Sauté over lively heat, just until the liver has lost its raw color. Then mix the onions and liver well, adding a handful of finely chopped parsley to the mix and seasoning well with salt and pepper. After a minute or so, remove the liver and onions to a warm serving plate.

Add a good slash of white wine to the skillet and deglaze, scraping up the sucs that will have formed with a wooden spoon or spatula. When the wine has reduced to a syrupy consistency, pour it over liver, sprinkle with some additional chopped parsley and serve immediately.