Classic Patty Melt

Method

1. Melt 2 tablespoons of the butter in a large skillet over medium-low heat, add the sliced onions, and cook for 20 minutes, stirring occasionally, until deeply golden and soft.
2. Stir the brown sugar into the onions during the last 2 minutes to help them caramelize further, then remove from the skillet and set aside.
3. In a small bowl, mix the mayonnaise, ketchup, and Dijon mustard together to make a quick special sauce.
4. Divide the ground beef into 4 equal portions and shape into thin oval patties about the same size and shape as your bread slices, since patty melts use flatter, wider patties than a bunned burger.
5. Season both sides of the patties with salt and pepper, then wipe out the skillet, place it back over medium-high heat, and cook the patties for 3 minutes per side until well browned and the internal temperature reaches 160F on a meat thermometer.
6. Remove the patties and set aside, then wipe the skillet clean and reduce the heat to medium.
7. Spread softened butter on one side of each of the 8 bread slices.
8. Place 4 bread slices butter-side down in the skillet, top each with a slice of Swiss cheese, a beef patty, a spoonful of caramelized onions, another slice of Swiss cheese, and a dollop of the special sauce.
9. Close each sandwich with the remaining bread slices, butter-side up, and cook for 3 to 4 minutes per side, pressing down gently with a spatula, until the bread is golden brown and crisp and the cheese is fully melted.
10. Remove from the skillet, let sit for 1 minute, then slice each sandwich in half and serve hot.