Heat a large skillet or griddle to medium high heat. Add the onions and a little bit of butter and season with salt and pepper. Cook until the onions are softened and begin to brown. Remove from the heat.
In a bowl, stir in your grated butter into your ground beef with a fork, to make sure the butter doesn’t melt. Season with salt.
Divide the beef into 4 loose rounds. Place the burgers on the skillet or griddle. 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! Season with salt and pepper. Once you see juices pooling on the meat, flip the burger. Add the cheese and cook for an additional minute. Once the cheese starts to melt, add your onions and put the top bun over the onions and cook for one more minute. Remove the patty with the bun from the heat and build your burgers with the bottom bun.