Set a large sauté pan, or skillet over medium heat. Add 4 tablespoons of butter to the skillet. Once melted, stir in the onion slices. Sprinkle the onions generously with salt and pepper. Turn the heat down to medium-low and stir the onions. Continue to stir and sauté the onions for 15 to 20 minutes, until they are soft and caramelized on all sides. Then turn off the heat.
Meanwhile, set out a large mixing bowl. Place the ground beef in the mixing bowl and add 2 tablespoons of Worcestershire sauce, ½ teaspoon of garlic powder, 1 teaspoon of salt, and ½ teaspoon of cracked black pepper. Crumble the ground beef with your hands, mixing the seasonings throughout the meat. Then press the ground beef into the bottom of the bowl and use a butter knife to divide into six equal portions.
Place a large sheet of parchment paper or foil on the countertop. One at a time, shape the ground beef portions into very flat ovals that are larger than your bread slices. (They will shrink as they cook.)
Set out 12 slices of thick-cut sandwich bread. Slather a generous teaspoon of yellow mustard over six of the slices.
Preheat a griddle, or a large skillet, to medium heat. (If using a skillet, you will most likely have to work in batches.) Carefully move the patties to the griddle or skillet. Sear the patties for 3 minutes per side.
Once you have flipped the patties over, place an equal portion of caramelized onions on top of each patty, spreading them out so the onions cover the meat entirely. Layer one slice of American cheese and one slice of Swiss cheese on top of each patty. Continue to cook until the hamburger patty has cooked for at least 6 minutes, total.
Now carefully move each loaded hamburger patty to the slices of bread with the mustard on top.
Wipe off the griddle with a paper towel. (It’s best to use a spatula to move the paper towel around the griddle, so you don’t burn your fingers.)
Now place 2 tablespoons of butter on the griddle and spread it around to cover the surface. Place the stacked patty melts on the griddle, and cover with the additional slices of bread.
Toast the bread for 2 to 3 minutes, until golden brown. Turn the heat down to medium-low if needed and watch them carefully.
Once they are ready to flip, add the final 2 tablespoons of butter to the griddle. Flip the patty melts over as the butter melts. Toast for another 2 to 3 minutes to make sure the second side is perfectly toasted.
Flip the patty melts onto a cutting board, making sure the cheese is on top of the patties. Cut them in half and serve immediately.
