01 -
Dump mayonnaise, ketchup, relish, and Worcestershire into a small bowl. Stir them together till creamy. Set it aside while you prep the rest.
02 -
Heat some butter in a big skillet over medium. Drop in the onion slices and cook them gently, giving them a stir every now and then, until they turn dark golden and taste sweet. This takes about 15 minutes. Take them off the heat when they're done.
03 -
Split the beef into 4 chunks and smash them into thin patties. Season each one with salt and plenty of black pepper on both sides.
04 -
Throw the patties into the pan on medium-high. Give them 3 or 4 minutes, then flip and cook the other side until they're nicely browned and cooked through. Move them to a plate when done.
05 -
Spread butter on one side of your bread slices. Lay half, butter-side down, in your clean skillet. Put Swiss cheese, beef patty, onions, and then cheddar cheese on each. Top off with another slice of bread, buttered side facing up.
06 -
Grill the built sandwiches over medium. Cook both sides, squishing a bit with your spatula, till the outside is crisp and cheese is gooey, about 2-3 minutes per side.
07 -
Let 'em sit for a minute before you slice. Serve hot, with a side of that creamy sauce.