JUICY LUCY BURGER

Steps:

• In a small bowl, stir together 2 teaspoons salt and 1 1/2 teaspoons pepper. Divide the beef into four equal portions, then halve each of those portions. Gently roll each portion into a ball using the palm of your hand, then flatten each into a disk shape on a large cutting board, pressing with your fingers and palm until just over 4 inches wide and 1/4-inch thick. Repeat to form eight patties. Season both sides of each patty with the salt-and-pepper blend.
• Add one slice cheese to the center of one patty, folding any overhanging corners of the cheese inward. Using a spatula, lift another patty from the cutting board and place it on top of the cheese, pressing gently to connect both patties. Gently pinch the edges of the patties together to thoroughly seal, then use your fingers to round the edges by pressing and patting along the perimeter. Repeat with the remaining patties, forming three more burgers, and set aside.
• In a large (preferably 12-inch) cast-iron skillet, melt butter over medium heat. Add onion; sprinkle with sugar, 1 teaspoon salt and 1/4 teaspoon pepper. Cook, stirring occasionally, adding 2 tablespoons water about every 5 minutes to prevent burning, until tender and caramelized, about 20 minutes. Once the onions are caramelized, transfer them to a bowl. (If you have any bits left in the pan, wipe them out.)
• Add the oil to the skillet to lightly coat the bottom and heat over medium-high until scalding-hot, about 2 minutes. Add the burgers and cook until seared and dark brown on the outside, 3 to 5 minutes per side. (You'll want to let the burgers cook undisturbed as long as possible to develop a crisp sear and to prevent them from splitting. Their insides will cook through by the time the outsides form a proper sear. Resist the urge to cut them open, as the cheese will spurt out.) Transfer burgers to a clean cutting board to rest.
• Meanwhile, toast the buns: Scrape up and remove from the pan any cheese that may have leaked, leaving a thin layer of accumulated fat in the pan. Reduce the heat to medium. Working in batches, toast the buns cut-side down until warmed and toasted in spots, about 1 minute. Form sandwiches by layering pickles, patties, then caramelized onions on buns. Serve immediately.