01 -
Heat a skillet or sauté pan over medium. Add half the butter (4 tablespoons) to the warm pan. Stir in the onion slices once the butter melts. Sprinkle in some salt and pepper for flavor. Lower the temperature to medium-low and keep stirring occasionally until the onions turn soft and golden, about 15 to 20 minutes. Turn off the heat and set them aside.
02 -
Combine the ground beef, Worcestershire sauce, garlic powder, black pepper, and salt in a big mixing bowl. Divide the meat into six equal portions, then press them into thin, oval-shaped patties. Make them a little bigger than the bread slices.
03 -
Take out 12 bread slices. Spread a thick layer of mustard on 6 slices.
04 -
Warm up a skillet or griddle on medium. Grill the burger patties for about three minutes a side. After flipping, layer each patty with some caramelized onions, a slice of Swiss cheese, and a slice of American cheese. In total, cook the patties for six minutes.
05 -
Take a mustard-covered bread slice and place a cooked beef patty on it. Top each patty with a plain bread slice.
06 -
Wipe the griddle with a paper towel and melt 2 tablespoons of butter on it. Grill each sandwich for around 2–3 minutes per side until golden brown. Add the remaining butter to the pan before flipping the sandwiches over.
07 -
Take the sandwiches off the griddle, cut them in half, and eat right away.