SERVES 2 TO 4 1 pound freshly ground beef (store-bought or homeground) 4 soft hamburger buns, preferably Martin’s Potato Sandwich Rolls 2 tablespoons unsalted butter, melted Vegetable oil Kosher salt and freshly ground black pepper 1 small onion, finely sliced (optional) 4 deli-cut slices American cheese Toppings as desired Divide the meat into 4 even portions and form into pucks about 2 inches high and 2½ inches wide Refrigerate until ready to use Open the buns but do not split the hinges Brush lightly with the butter, then toast under a hot broiler or in toaster oven until golden brown, about 1 minute Set aside Using a wadded-up paper towel, rub the inside of a 12inch heavy-bottomed stainless steel or cast-iron skillet with vegetable oil, then heat over medium-high heat until just beginning to smoke Season the beef pucks on the top with salt and pepper, then place seasoned side down in the skillet Using a wide heavy spatula, press down on each one until it is roughly to 4½ inches in diameter and ½ inch thick; it helps to use a second spatula to apply pressure on the first one Season the tops with salt and pepper Cook, without moving the burgers, until a golden brown crust develops on the bottom, about 1½ minutes Use the edge of the spatula to carefully scrape up and flip the patties one at a time, making sure to get all the browned bits If using onions, add to the tops of the burgers, then cover each with a cheese slice Continue to cook until the patties are the desired doneness—about 30 seconds longer for medium-rare Top the buns and/or patties as desired, transfer the patties to the buns, close the burgers, and serve ... pressure on the first one Season the tops with salt and pepper Cook, without moving the burgers, until a golden brown crust develops on the bottom, about 1½ minutes Use the edge of the spatula to carefully scrape up and flip... Use the edge of the spatula to carefully scrape up and flip the patties one at a time, making sure to get all the browned bits If using onions, add to the tops of the burgers, then cover each with a cheese slice Continue to cook until the patties are the desired doneness—about 30... with vegetable oil, then heat over medium-high heat until just beginning to smoke Season the beef pucks on the top with salt and pepper, then place seasoned side down in the skillet Using a wide heavy spatula, press down on