Patty Melt

Thinly slice 150 g onion and melt 2 tablespoons butter in a skillet over low heat. Stir often at first as the onion releases moisture. Continue for at least 20 minutes, lowering the heat further if the bottom dries or the edges begin to catch, until the onion is dark brown, soft, and distinctly sweet. Divide 280 g ground beef into two portions. Shape each into a thin patty slightly larger than the bread, allowing for shrinkage and keeping the beef in proportion to the sandwich. Season both sides with 0.75 teaspoon salt and 0.5 teaspoon black pepper. Heat the skillet over medium-high and cook the patties for 2 to 3 minutes per side. Remove them when the edges are well browned and the juices no longer look raw red. Spread 2 tablespoons mayonnaise across 4 bread slices. Assemble the sandwiches with 100 g cheddar, the hot patties, and the caramelized onions, arranging cheese against the beef and onions so it melts around and binds the filling. Return the sandwiches to the skillet over medium-low heat. Press gently and cook each side for 3 to 4 minutes. The slower final heat gives the cheddar time to melt before the bread burns. Remove them when both faces are deeply browned and crisp, rest the hot filling briefly, and then slice.