You used too much cooking oil, you do not need to dip the liver in anything before you dredge it in flour. Cut the liver into smaller pieces, 1/2 inch thick is too thick, an eight to a quarter inch is better then dredge and put in a light coating of oil in the skillet, not a quarter of the way up the side, you are not deep frying. The thicker you cut it the longer you must cook it and the drier it will be once the liver is browned on one side add the onion slices, also thin, turn the liver and cook both together flipping and stirring until the onions are soft. and it will be f=done just right, use a medium heat not high heat, forget the butter, that makes no sense. Flour will stick to liver, you and not making a breading in the liver just something to soak up the juices. Also salt and pepper your dredging flour so the liver will be seasoned. You are making something simple way to complicated.