Use your hands to shape the beef into 4 balls. Set aside.
Peel and thinly slice onions using a mandoline or sharp knife.
Heat a lightly oiled cast iron skillet over medium heat.
Toast the buns face-down in the skillet until golden brown. Set aside.
Pile a small mound of onions in the hot skillet over medium heat. Top with a ball of beef.
Season beef generously with salt and pepper.
Use two spatulas to press firmly on the beef to flatten into a thin disk. (You can optionally place a square of parchment paper on top of the beef before pressing.)
Cook until browned on the edges, then flip.
Immediately top with a slice of cheese.
When the burger is cooked through and the cheese is melted, transfer to the bottom of a toasted bun. Repeat the cooking process with all beef, onions, and cheese.
Top the burger with sliced tomatoes, pickles, and lettuce.
Spread special burger sauce or condiment of choice over the top bun, if desired.
Place the top bun on top of the smash burger and serve immediately.