01 -
Warm up a large pan on medium-low heat and melt 2 tablespoons of butter. Toss in those onions and stir them around every so often until they're super soft and golden—plan for about 30 minutes.
02 -
Blend together the mayo, mustard, black pepper, ketchup, and garlic powder until smooth. Put it aside for later.
03 -
Take your cast iron skillet and heat it on medium-high heat. Drop 2 tablespoons of butter in there, let it melt, and then cook the patties just like the package says. Clean out the skillet when you're done.
04 -
Spread your sauce over each of the 8 bread slices. Pile some of those caramelized onions onto 4 of them. Follow up with a burger patty, one piece of cheddar, one slice of Swiss, and cap it off with the other 4 bread slices.
05 -
Pop 2 tablespoons of butter in a pan over medium-low heat and move it around until melted. Lay the sandwiches in the pan and cook until the bread goes golden. Flip 'em, toss the last 2 tablespoons of butter into the pan, and wiggle the sandwiches a bit to soak it all up. Cook until both sides are nice and brown, then serve them warm.