Place the beef balls on the hot skillet or griddle, leaving room between each. Immediately top each ball with a generous handful of sliced onions.
Season and smash. Sprinkle each mound with kosher salt and black pepper. Using a stiff spatula or burger press, smash each ball into a thin patty, about ¼–⅓ inch thick. Press firmly once and don’t press again.
Cook the first side. Let the patties cook undisturbed until the edges of the onions and meat are browned and crisp, about 2–3 minutes.
Flip, add cheese, and steam buns. Slide a thin metal spatula under each patty, scraping up any browned bits, and flip so the onions are now touching the pan. Immediately top each patty with a slice of American cheese. Stack the bun tops, then the bottoms, cut-side down, on top of each patty to steam.
Finish cooking. Cook another 2–3 minutes, until the cheese is melted, the onions are soft and browned, and the patties are cooked through.