Step 1

Heat a large cast-iron skillet over medium heat. Working in batches as needed, place 4 split buttered hamburger buns cut-side down in the skillet and toast until golden brown on the bottom, 1 to 2 minutes. Transfer to plates or a serving platter cut-side up. Turn off the heat.

Step 2

Divide 1 pound lean ground beef into 4 portions (4 ounces each). Gently roll each one into a ball and press into a 3-inch wide patty. Season all over with 1 teaspoon kosher salt and 1/4 teaspoon black pepper. Divide 1 very thinly sliced medium onion between the patties (they will be piled high).

Step 3

Heat the same skillet over high heat until it feels very hot when your hand hovers over it (don’t get too close!). Place 2 of the patties in the skillet onion-side up, spacing them evenly apart. Use a non-slotted flat metal spatula to press the patty as thin as you can, pressing in different spots to ensure it’s as flattened as possible and the onions are pressed in. Cook until browned on the bottom and gray around the edges, 2 to 3 minutes.

Step 4

Carefully flip the patties, making sure to scrape up the crust and griddled onions from the bottom. Top each patty with 1 American cheese slice. Cook until the second side is browned and the cheese is melted, 1 to 2 minutes. Transfer to 2 of the bottom bun halves. Scrape up any remaining onions and add them on top of the patties.

Step 5

Repeat cooking the remaining 2 patties. Top the burgers with the desired toppings, close each one with a top bun, and serve immediately.