Preheat your grill to a medium, direct heat. Place a large griddle or cast iron pan in the grill.
Thinly slice the onion on a mandoline – like so thin they only have one side. Divide the beef into four equal parts and roll them into meatballs. Season with plenty of kosher salt.
Lightly oil the pan and place the burgers on the pan. Once juices start to pool at the bottom of the burger use a smash burger press or large metal spatula to smash the burger until it is SUPER THIN! Add the onions.
Once you see juices pooling on the meat flip the burger. Add the cheese and place a bottom bun on top of the four patties. As soon as the cheese is melted pull them off.
Toast the bottom buns on the still hot griddle then smear with mayo. Top with pickles and the onion burger patty.