Before we get too deep into this recipe let's talk about a couple of options with the Liver itself. I always rinse the liver off with cool water and cut into "normal" pieces (sometimes they come in massive slabs). If you are eating liver to help boost your iron level (or of anyone in your family), you're going to want to skip this step - I'll tell you why in a minute. Take a shallow pan and pour milk into it, place the liver in it to soak for at least ten minutes. Back to the iron - you want to skip the soaking step because, while it takes some of the "funk" out of the liver, it also takes out the iron that you're trying to boost.
While the liver is soaking or resting, add sliced onions (A LOT - they'll cook down) to a hot pan with butter in it. Sautee until starting to brown and soften. Place in an oven-safe dish, put in oven on lowest temp you can to keep the onions warm while you cook the liver.
Add enough Vegetable Oil to the onion pan to thoroughly cover the bottom.
While oil is heating, add Salt & Pepper to Flour in a shallow dish or pie plate. Dredge Liver in Flour mixture, shake off excess flour & fry on medium-high heat until edges start to brown (3-5 minutes). Flip liver over and cook until second side is browned (an additional 3-5 minutes). Remove to a pan or cookie sheet and place in oven to keep warm while you cook the rest of the liver.
Serve liver with a big pile of onions on top of it and serve it warm.