Place a pan over medium-low heat and melt 1 tablespoon of butter. Add the thinly sliced onions and a pinch of salt. Cook them slowly, stirring occasionally, until they turn a deep golden brown and become soft and sweet. This usually takes about 10 to 12 minutes. Once finished, remove the onions from the pan and set them aside.

While the onions cook, divide your ground beef into four equal portions. Roll them into loose balls; do not overwork the meat or it will become tough. In a small bowl, whisk together the mayonnaise, Dijon mustard, hot sauce, and garlic powder to create your signature sauce. Lightly butter one side of each bread slice.

Wipe out your skillet or heat a cast-iron griddle over high heat. Add a tiny bit of oil. Place the beef balls onto the hot surface. Use a heavy spatula or a burger press to smash them down as flat as possible. Season generously with salt and pepper. Cook for 2 minutes until the edges are dark brown and crispy. Flip the patties, add a slice of cheese to each, and cook for another minute.

Spread a layer of the secret sauce on the unbuttered side of your bread slices. Place one patty on a slice of bread, top with a generous portion of caramelized onions, and then add the second patty. Close the sandwich with another slice of bread, buttered side facing out. Place the sandwich back into the skillet over medium heat. Grill both sides until the bread is perfectly golden and the cheese is fully melted.

Remove the patty melt from the heat and let it rest for 60 seconds. This allows the juices to redistribute and the cheese to set slightly. Slice the sandwich diagonally to show off the layers and the crispy beef edges. Serve immediately while it is hot and crunchy.