Prep the Onions:
Slice the onion as thin as possible. You want a pile of paper-thin onions—these cook quickly and create that sweet caramelized layer under the burger.

Form the Meat Balls:
Divide the ground beef into 4 equal portions and roll each into a loose ball. Do not season yet.

Heat the Griddle or Skillet:
Preheat a cast iron skillet or griddle over medium-high heat. Lightly oil or butter the surface.

Smash the Burgers:
Place a pile of onions directly onto the hot griddle. Immediately top with one beef ball and smash it down hard with a spatula or burger press until thin and flat. Press the onions into the beef. Season generously with salt and pepper.

Cook Until Crispy:
Let the burger cook, untouched, for 2–3 minutes until the edges are brown and crispy. Flip the patty so the onions are on the bottom and cook for another 1–2 minutes. The onions will caramelize and stick beautifully to the patty.

Add Cheese:
Add a slice of cheese on top during the last minute of cooking. Cover the burger with a lid or metal bowl to help the cheese melt, if desired.

Toast the Buns:
Lightly toast the buns face-down on the griddle or in a separate skillet until golden.

Assemble:
Place the onion-smashed patties on the bottom buns, then add your favorite toppings like pickles and mustard. Top with the other half of the bun and serve immediately.