Add the ground chuck to the cast iron skillet, and let it sear, and cook for a couple of minutes. Then, with a large spatula smash the burger down, completely flattening it. Let cook another minute, then flip it over. You should have an awesome, caramelized burger. Season with a pinch of salt and pepper, then add the thinly sliced onions over the top of the burger, letting the burger steam the onions. After a couple of minutes, flip the burger, along with the onions, over, allowing the onions to cook.