Smash burgers get their name from the act of smashing the patties into a griddle — or, in this case, a cast iron skillet — so you maximize the amount of meat that caramelizes where it touches the surface, creating crispy edges. You can make these on an outdoor grill, but you still need to use a cast iron pan, since the amount of pressure needed to press the patties would force the meat to fall through the grill grates otherwise.
Place a large plate or a baking sheet on a work surface. Divide the beef into 8 equal portions, then loosely shape each portion into a ball. Flatten one ball between your palms — it should be three to four inches in diameter and about ¼-inch thick — and set the patty on your plate or baking sheet; repeat making the other patties. At this stage, you can stack the patties, with squares of parchment paper or foil between each patty, and chill until you’re ready to cook.
Heat a heavy skillet over high heat for 5 minutes, or place the skillet on a gas grill over high heat or on a charcoal grill set up for direct grilling. While it heats, season four of your patties liberally with salt and pepper. (We’re cooking the patties in batches, so season only what you’re about to cook or else the salt will draw out moisture on the other patties as they sit, causing the burgers to get wet and stick to the pan.)
Once the pan is hot (you should see small wisps of smoke swirling up off the bottom), arrange the four patties evenly over the bottom of the skillet, seasoned side down. Set a timer for two minutes and don’t touch them. Take this time to season the patty sides facing up with salt and pepper. After two minutes, flip the patties, set a timer for two minutes again, and cook, using your flipping implement to press firmly onto the patties, smashing them into the skillet to flatten.
If you want cheeseburgers, press for the first minute of cooking on this second side, then quickly top each patty with a slice of cheese and cover the skillet with a lid (or close the grill) to help melt the cheese on the patties. Transfer your patties to a clean plate and tent with foil to keep them warm while cooking the remaining four patties, using the remaining 4 cheese slices, if using.
While the patties rest, remove the skillet from the heat source. Immediately place the buns, cut sides down, in the skillet and toast until golden brown and crusty. Stack four patties on top of the other 4 patties, then transfer each stack to a bottom bun. Add one lettuce leaf, tomato slice and onion slice on each patty stack, then add mayonnaise, mustard, ketchup and/or pickle chips to serve.