Heat a cast-iron skillet or griddle over the highest heat until smoking. Place 2 meat balls in the pan, scatter a pinch of diced onion on each, and immediately smash flat with a burger press or spatula. Season with flaky salt and cook 2 minutes without moving.

Flip each patty, lay a cheese slice on top, and cook 1 more minute. Stack two patties per bun. Repeat for remaining burgers.

Spread special sauce on both bun halves, add lettuce, tomato, and double patty stack. Serve immediately.