Step 1

Melt 2 tablespoons of butter in a large skillet over medium heat. Add the sliced onion and cook for 20 to 25 minutes, stirring occasionally, until the caramelize and are a deep brown color. Keep an eye on them to not burn them, turn down the heat to medium low if needed. Transfer the onion to a plate and wipe the skillet clean.

Step 2

Season the ground beef with salt and pepper and shape it into 4 patties that are roughly the size of your slices of bread.

Step 3

Add the patties to the skillet, do it in batches if needed, and cook for 3 minutes per side over medium-high heat. Transfer the patties to a plate and set aside.

Step 4

Wipe the skillet clean again and add 1 tablespoon of butter. Once the butter melts add 4 slices of bread and toast until golden brown. Remove 2 slices and flip the other two remaining slices.

Step 5

Place a slice of cheese over each piece of bread in the skillet, then top each with a patty and another slice of cheese. Spread a tablespoon of mayo on the other two toasted bread, on the side that's untoasted, and place the mayo side over the sandwiches in the skillet.

Step 6

Turn the heat to medium-low and cook the sandwiches for 3 to 4 minutes, flipping them halfway through until nicely toasted and the cheese is melted.

Step 7

Remove the patty melts from the skillet and repeat with remaining ingredients.

Step 8

Slice sandwiches in half and serve immediately with chips or french fries.