Preheat a cast iron skillet or griddle over medium heat to 375°F. Add a spoonful of beef tallow and spread it around the cooking surface with the flat side of a spatula.

Place the ground chuck in a mixing bowl and shape it into 2.7 to 3 ounce balls using a portion scoop or your hands. Do not season the meat yet.

Set the beef balls onto the hot cooking surface, leaving 2 to 3 inches between each one. Season each ball with kosher salt immediately before smashing.

Press a handful of the thinly sliced onions onto the center of each meatball so they stick to the beef.

Using a stiff metal spatula, forcefully smash each ball into a thin patty. Let the edges push out into ragged, lacy strands. Do not worry if the patty seems too thin; it will shrink while cooking. Avoid tearing holes in the meat. Once smashed, leave the patties alone until moisture begins to pool on the top.

Flip the patties so the onions are against the cooking surface, then lay a slice of American cheese on each patty. Split the potato rolls and place the top crown directly onto the cheese, then set the bottom heel upside down on top of the crown to let it ride and steam.

Cook for another 2 to 2.5 minutes until the cheese has melted and the bottoms are browned. For a double burger, set the patty and bun stack on top of a second cooked patty. Use the spatula to lift the whole stack, remove the heel from the bottom, and transfer the patties and crown onto the waiting heel on a serving plate.

Add pickle slices or special sauce if desired and serve the burgers immediately.