In your cast iron skillet, caramelize your onions. Remove when deeply golden brown and set aside
While the onions are cooking, slit your ground beef into 2 patties and shape them into squares slightly bigger than the size of your bread (they will shrink when they cook).
Generously season both sides of your meat. If you don’t have the Kinder’s rub I use, I suggest mixing equal parts salt, pepper, and garlic powder, and using that! Add the burgers to your pan and cook to medium rare (about 2.5-3 mins per side). Make sure to only flip once to lock in juices. (You can cook longer if needed, but the meat will cook more later.)
Remove the meat and set aside. Wipe out the pan if there is a lot of grease.
Butter one side of the bread and place it on the pan, butter side down.
Add a piece of cheese, then some of the onions, then a cooked burger patty, 1 more slice of cheese, and the final piece of bread (buttered with butter facing up).
Cook until deeply golden brown and flip. Do the same for the other side.
Slice and eat.
Enjoy!