Step 1

Begin by shaping the ground beef into two patties, ensuring they are slightly larger in diameter than the bread slices. Season the patties with salt and pepper.

Step 2

In a skillet over medium heat, melt a tablespoon of butter and add the sliced onions. Cook the onions for about 10-12 minutes, stirring occasionally, until they become golden brown and caramelized. Once done, transfer the onions to a plate.

Step 3

In the same skillet, cook the beef patties for about 3-4 minutes per side, or until they reach your desired level of doneness. For medium-rare, aim for an internal temperature of 130-135°F (54-57°C).

Step 4

While the patties are cooking, spread a thin layer of mayonnaise on one side of each bread slice. On the opposite side of two bread slices, spread a mixture of ketchup and Dijon mustard.

Step 5

Once the patties are cooked, remove them from the skillet and set aside. Wipe the skillet clean and return it to medium heat.

Step 6

Assemble the patty melt by layering a slice of Swiss cheese on top of the mustard-ketchup side of two bread slices. Place a cooked beef patty on top of the cheese and divide the caramelized onions evenly between the two patties. Top each patty with another slice of Swiss cheese, and then cover with the remaining two bread slices, mayonnaise-side down.

Step 7

Melt a tablespoon of butter in the skillet and place the assembled sandwiches in the pan. Cook for about 2-3 minutes per side, or until the bread turns golden brown and the cheese is melted.

Step 8

Once the sandwiches are nicely grilled and the cheese is gooey, remove them from the skillet. Allow them to cool slightly before slicing and serving.