01 -
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 with salt and pepper. Turn heat to medium-low and cook, stirring often, for 15–20 minutes until soft and caramelized. Turn off the heat.
02 -
In a large bowl, mix ground beef with Worcestershire sauce, garlic powder, salt, and black pepper. Divide into six equal portions and shape into thin, large oval patties slightly larger than the bread slices.
03 -
Lay out 12 slices of bread. Spread a generous teaspoon of yellow mustard on six of the slices.
04 -
Preheat a griddle or skillet to medium heat. Cook the beef patties for 3 minutes per side. After flipping the patties, top each with caramelized onions, one slice of American cheese, and one slice of Swiss cheese. Cook for a total of 6 minutes.
05 -
Place each cooked patty on a mustard-slathered slice of bread. Top with another slice of bread.
06 -
Clean the griddle with a paper towel and melt 2 tablespoons of butter on the surface. Toast the sandwiches for 2–3 minutes per side or until golden brown. Add the remaining butter when flipping the sandwiches.
07 -
Remove from the griddle, slice in half, and serve immediately.