Thinly slice the yellow onion using a sharp knife or mandoline until the slices are almost translucent. Separate the slices and set aside.

Divide the ground beef into four loose balls (about 4 ounces each). Do not press or compact the meat.

Heat a cast iron skillet or flat-top griddle over high heat until very hot.

Place a small pile of thinly sliced onions on the hot skillet, roughly the size of a burger patty.

Place a beef ball directly on top of the onions. Using a sturdy metal spatula, smash the beef firmly into the onions until the patty becomes very thin.

Season the top with salt and black pepper. Cook undisturbed for about 2–3 minutes until the edges become crispy and golden brown.

Scrape under the patty with a metal spatula and flip the burger so the onions are now on top.

Add a slice of American cheese and cook for another 1–2 minutes until the cheese melts.

Toast the burger buns in butter on the skillet until lightly golden.

Place the cooked Oklahoma Onion Smash Burger on the toasted bun, add optional toppings if desired, and serve immediately.