Well, if you already have conflicting opinions, the last thing you may want is ours.
But before we get to that, searing never seals in juices. It may concentrate flavors at the surface of the meat through a process some people mistakenly refer to as caramelization, but which people who really want to impress you with their food knowledge call Maillard reactions. (It is the surface browning of meat and bread that occurs when the temperature rises above 285° F (140°C)). But sear a steak or another piece of meat, cook it until it’s done the way you like it, and then set it on a cutting board. A minute and a half later, when juices begin to collect on the board, you will be compelled to agree that searing did not and does not to seal in meat juices.
Our inclination is not to cook liver fast and furiously, but there’s no reason to dally either. We’d cook it over medium-high heat. You can cut it into half-inch strips before you cook it, if you like. If you don’t, you should make 1/8th-inch cuts at 1-inch intervals all around the outside edge to help keep it from curling as it shrinks. If the butcher hasn’t already done so, remove the membrane that covers the liver before you cook it.
And remember, it’s the onions that want the long, slow cooking.