Divide meat of your choice into 4–5 pieces (approximately 3–4 ounces each), rolling each gently into a little ball. I like to do this on a sheet tray, especially if doing a lot at once.
Meanwhile, heat a large cast iron skillet over high heat until very, very hot.
Toast bun in skillet until lightly golden. Heat the bottom of the spatula for a few seconds on the hot pan, then add patty (two if there’s room) to the skillet and smash it with the spatula until it’s very thin. Cook, without moving it, until it’s very crispy and browned. Add some onions to the patty, press them in with the spatula, then flip. Add a slice of American cheese to the top of each patty and continue to cook until the cheese is melted and the onions on the bottom are nice and griddled. Repeat this process with the remaining patties. If you’re making a lot of burgers, carefully wipe your skillet between batches to avoid any super burnt pieces.
Assemble the burgers to your liking. The HM order of operations is mustard on bottom bun, pickles, patty, more raw onion on top, sheet of iceberg, top bun.
