The onions go under the patty and fry into it. Thirty seconds too long is better than thirty seconds too short.
Divide the beef into four loose balls. Do not compact them, do not season them yet. Chill until you are ready.
Get a heavy cast iron skillet or griddle as hot as it will go. Toast the cut buns in a dry corner of it and set aside.
Lay a small pile of sliced onion on the hot metal, put a ball of beef on top, and smash it flat and thin with a stiff spatula. Hold for ten seconds so it takes the shape.
Salt the top. Leave 90 seconds until the edges are lacy and dark brown, then scrape under it firmly — that crust is stuck to the pan and it is the entire dish.
Flip, cheese on immediately, 30 seconds more. Stack onto the bun with mustard and pickles while everything is still too hot to handle.