Gently divide the ground chuck into four 4-ounce balls, being careful not to overwork the meat. Season generously with salt and pepper. Using a mandoline or sharp knife, slice the onion as thinly as possible. If making the sauce, combine all sauce ingredients in a small bowl and stir until smooth.

Place a large cast-iron skillet or griddle over high heat. Allow it to get screaming hot, which should take about 5-7 minutes. Do not add any oil.

Place two beef balls on the hot skillet, leaving plenty of space between them. Immediately top each with a very large handful of the thinly sliced onions. Place a piece of parchment paper over one onion-topped patty and use a sturdy spatula to smash it down as flat as possible, holding for 10 seconds to create a good crust. Repeat with the second patty.

Cook for 2-3 minutes, until the edges are dark brown and crispy. Using the edge of your spatula, carefully scrape underneath the patty to lift it off the skillet, ensuring all the crispy bits and onions come with it. Flip the burger. The onions should now be on the bottom, continuing to caramelize.

Immediately after flipping, place a slice of American cheese on top of each patty. Cook for another 1-2 minutes, until the cheese is perfectly melted and the burger is cooked through.

Remove the burgers from the skillet and let them rest. Place the buns, cut-side down, onto the skillet to toast in the rendered beef fat for about 30-60 seconds until golden brown. Spread the burger sauce on the buns, add the patties, and serve immediately.