01 -
In a large skillet, melt 2 tablespoons of butter over medium-low heat. Add the onions and cook over low heat, stirring occasionally, until they are golden and soft—about 30 minutes.
02 -
Make the sauce by combining the mayonnaise, ketchup, mustard, garlic powder, and black pepper. Set aside.
03 -
Heat a large cast iron skillet over medium-high heat. Add 2 tablespoons of butter, then cook the burger patties according to the package instructions. Wipe the skillet clean.
04 -
Spread the prepared sauce on one side of all 8 slices of bread. Divide the caramelized onions between 4 slices of bread. Top each slice with a burger patty, one slice of cheddar cheese, one slice of Swiss cheese, and a second slice of bread.
05 -
Add 2 tablespoons of butter to a skillet over medium-low heat and swirl to coat the pan. Add the assembled sandwiches and cook until the bread is golden brown, then flip the sandwiches. Add the remaining 2 tablespoons of butter to the skillet, moving the sandwiches around to coat them evenly in butter. Cook until the second side is golden brown. Serve immediately.