Place a heavy skillet over medium-high heat. Leave it there for 5 minutes. Loosely gather a handful of ground beef and slip it into the pan. Repeat 6 times. The burgers should be freeform lumps.
With the side of a spatula, push the burgers into a semblance of a round. After about a minute, pile on all the onions and then, with as much force as you can manage, smash the onions into the meat. Again, work the sides of the patties into a semblance of a round. Cook another minute and then flip. Season with salt. Smash the burgers again, hard.
Place burger on bottom half of the bun and spread with mustard. Spread the cheese on top half of the bun and place on the burger.
Recipe courtesy of John T. Edge, Burgers: An American Story, Putnam 2005