Worcestershire sauce, ½ teaspoon of garlic powder, 1 teaspoon of salt, and ½ teaspoon of crushed black pepper to the mixing bowl with the ground beef. Use your hands to crumble the ground beef, incorporating the spices as you go. Once the ground beef is pressed into the bowl’s base, divide it into six equal sections with a butter knife.
Lay down a big piece of foil or parchment paper on the worktop. Make huge, flat ovals out of each batch of ground beef, bigger than your bread slices. (Culinary time causes them to shrink.)
Prepare a dozen thick-cut sandwich slices. Evenly distribute a teaspoon of yellow mustard over six of the cucumber slices.
Set a big skillet or griddle over medium heat to get it hot. (It is possible that you will need to work in batches if you are using a skillet.) Bring the patties to a boil on a pan or griddle. Grill the patties for three minutes on each side.
After turning the patties over, evenly distribute the caramelized onions over the top of each burger, being sure to coat the meat completely. On top of each burger, layer one piece of Swiss cheese and one slice of American cheese. Keep cooking the hamburger patties for a total of six minutes.
Gently place each filled hamburger patty onto the mustard-topped bread pieces.
Use a paper towel to dry the griddle. (To avoid burning your fingers, it’s ideal to use a spatula to transfer the paper towel from one area of the griddle to another.)
Next, coat the griddle with 2 tablespoons of butter by spreading it out evenly. Top the stacked patty melts with the extra bread pieces and set them on the griddle.
Brown the bread in a toaster oven for two or three minutes. If necessary, reduce the heat to medium-low and keep a close eye on them.
Before you turn them, add the last two teaspoons of butter to the griddle. As the butter melts, turn the burger over. Be sure the opposite side is equally toasted by toasting it for an additional 2–3 minutes.
Transfer the burgers to a chopping board and turn them over so the cheese is facing up. Serve right away after halving.
