Heat a skillet over medium-low, add butter and cook onions until deep golden and sweet, about 20–25 minutes; stir occasionally.

Divide beef into 4 equal balls, season lightly with salt and pepper.

Heat a heavy skillet or griddle over high with oil until shimmering.

Place a beef ball on the hot surface and smash thin with a spatula; cook 2–3 minutes until edges brown.

Flip, add cheese, cook another 1–2 minutes.

Toast buns on the griddle briefly, assemble with onions and condiments.