Form the meat into 36 golf ball-size balls. Pulse the onion in a food processor until finely chopped.
Heat a cast-iron skillet over high heat. Working in batches, add the meatballs to the skillet and smash them very flat with a spatula. Season with salt and pepper, then sprinkle the onion over the patties. Cook 1 minute, then flip and smash the burgers with the spatula again. Cook 1 more minute for well done; place both halves of the buns on top of the patties to steam for the last few seconds of cooking.
Serve the burgers on the buns; top with pickles, ketchup and mustard.